Baker started as an apprentice at Baker & Botts on December 1, 1877, reading law in his spare time and performing general office duties for the firm.
He obtained a licence to practice law in Texas two years later, and was promoted to partner of Baker, Botts & Baker on July 1, 1881.

Baker established a niche at the firm by specializing in trial law on behalf of railroad clients, seeking new clients emerging from the growing local economy, and handling the business interests of the firm's top individual client, William Marsh Rice.
The native New Englander profited from the Civil War by shipping Texas goods and supplies over the Gulf through Matamoros, Mexico, where wagon caravans provided ground transportation for Texas freight.
After the war, Rice amassed an empire that he managed from his primary residence in New York City.
Baker's father acted as an advisor and liaison for managing Rice's broad portfolio of assets along the western Gulf coast, though later Baker himself managed Rice's portfolio.
In Houston alone, Rice owned large stakes in a bank, a brewer, a brick works, a gas light utility, and a seed oil refiner.
Baker's legal work led to several leadership positions within these companies.
He was vice-president of the Merchant's & Planters Oil Company, and he served as a director for at least three others.
In addition to representing Rice's interests in local companies, Baker gained other executive positions.
He acted as vice president of the Texas Rolling Mills from 1887.
He was president of the Houston Abstract Company and the Citizens Electric Light & Power Company.
He served as director for the Houston Gas Light Company and the Houston National Bank.

By the end of 1889, Baker was able to bring cases to the Supreme Court of Texas.
By 1892, Baker started taking over his father's business practice while relinquishing some of his railroad clients to new attorneys.
Two years later he had virtually replaced his father at the firm, which by that time was known as Baker, Botts, Baker & Lovett.
One of the partners, Walter Browne Botts, died later in 1894, and Baker became the legal guardian of Tom Botts, who was Walter's son and a descendant of one of the firm's founders.
Through 1900, Baker was a fifty percent partner in the firm.

During this period, Baker defended Rice's estate when a new will of his late wife surfaced.
Elizabeth "Libbie" Baldwin Brown Rice died on July 24, 1896.
She had drawn two simple wills in 1888 and 1892.
The second of these left most of her estate to her husband, but neither will suggested that she had great personal wealth.
Though Texas was a community property state, William and "Libbie" married in 1867 after Rice had already built most of this fortune.
Their primary residences were New York City and Dunellen, New Jersey, but they also maintained an apartment at the Capital Hotel in Houston, where they spent many winters.
In April 1896, Libbie was in poor health and she sought warmer climate as a remedy, bringing the couple to Houston.
She developed a friendship with a local lawyer and his wife, Orren T. and Marian Seward Holt, while also entertaining a niece from Cleveland, Mamie Baldwin Huntington.
Libbie suffered a stroke the next month, and she died in Wisconsin two months after.
During her illness in Houston, Holt wrote a new will for her, naming himself as sole executor and designating ten percent of the proceeds as his commission, which was witnessed only by two of his in-laws.
The 1896 will assumed $2 million in community property, and designated $200,000 to Huntington, and another $150,000 to Huntington's close relations.
Rice had not known about any of his wife's wills, but he was so informed by Baker in September.
Two weeks later Baker drew a new will for Rice, which bequeathed the bulk of his estate to the foundation for the Institute for the Advancement of Literature, Science, and Art.
Along with Judge John Bartine, Baker was an executor.
Holt filed Libbie's 1896 will in Harris County on March 22, 1897.
Baker and his firm represented Rice.
Holt prevailed in the probate hearing as
Baker started as an apprentice at Baker & Botts on December 1, 1877, reading law in his spare time and performing general office duties for the firm. He obtained a licence to practice law in Texas two years later, and was promoted to partner of Baker, Botts & Baker on July 1, 1881.
Baker established a niche at the firm by specializing in trial law on behalf of railroad clients, seeking new clients emerging from the growing local economy, and handling the business interests of the firm's top individual client, William Marsh Rice. The native New Englander profited from the Civil War by shipping Texas goods and supplies over the Gulf through Matamoros, Mexico, where wagon caravans provided ground transportation for Texas freight. After the war, Rice amassed an empire that he managed from his primary residence in New York City. Baker's father acted as an advisor and liaison for managing Rice's broad portfolio of assets along the western Gulf coast, though later Baker himself managed Rice's portfolio. In Houston alone, Rice owned large stakes in a bank, a brewer, a brick works, a gas light utility, and a seed oil refiner. Baker's legal work led to several leadership positions within these companies. He was vice-president of the Merchant's & Planters Oil Company, and he served as a director for at least three others. In addition to representing Rice's interests in local companies, Baker gained other executive positions. He acted as vice president of the Texas Rolling Mills from 1887. He was president of the Houston Abstract Company and the Citizens Electric Light & Power Company. He served as director for the Houston Gas Light Company and the Houston National Bank.
By the end of 1889, Baker was able to bring cases to the Supreme Court of Texas. By 1892, Baker started taking over his father's business practice while relinquishing some of his railroad clients to new attorneys. Two years later he had virtually replaced his father at the firm, which by that time was known as Baker, Botts, Baker & Lovett. One of the partners, Walter Browne Botts, died later in 1894, and Baker became the legal guardian of Tom Botts, who was Walter's son and a descendant of one of the firm's founders. Through 1900, Baker was a fifty percent partner in the firm.
During this period, Baker defended Rice's estate when a new will of his late wife surfaced. Elizabeth "Libbie" Baldwin Brown Rice died on July 24, 1896. She had drawn two simple wills in 1888 and 1892. The second of these left most of her estate to her husband, but neither will suggested that she had great personal wealth. Though Texas was a community property state, William and "Libbie" married in 1867 after Rice had already built most of this fortune. Their primary residences were New York City and Dunellen, New Jersey, but they also maintained an apartment at the Capital Hotel in Houston, where they spent many winters. In April 1896, Libbie was in poor health and she sought warmer climate as a remedy, bringing the couple to Houston. She developed a friendship with a local lawyer and his wife, Orren T. and Marian Seward Holt, while also entertaining a niece from Cleveland, Mamie Baldwin Huntington. Libbie suffered a stroke the next month, and she died in Wisconsin two months after. During her illness in Houston, Holt wrote a new will for her, naming himself as sole executor and designating ten percent of the proceeds as his commission, which was witnessed only by two of his in-laws. The 1896 will assumed $2 million in community property, and designated $200,000 to Huntington, and another $150,000 to Huntington's close relations. Rice had not known about any of his wife's wills, but he was so informed by Baker in September. Two weeks later Baker drew a new will for Rice, which bequeathed the bulk of his estate to the foundation for the Institute for the Advancement of Literature, Science, and Art. Along with Judge John Bartine, Baker was an executor. Holt filed Libbie's 1896 will in Harris County on March 22, 1897. Baker and his firm represented Rice. Holt prevailed in the probate hearing as 
James A. Baker (born 1857)