He joined the Seventeenth Infantry Regiment of the United States Army as a Second Lieutenant in 1899, after the United States had occupied the Philippine Islands in the Spanish–American War.
In the course of nearly four years in the Philippines, he served under General Arthur MacArthur, Jr., was on the staff of General Leonard Wood during the Moro rebellion, and commanded a battalion of Philippine Scouts.
He resigned from the Army in 1905 with the rank of Captain, and became New York's First Deputy Commissioner of Police in January 1906, at the age of 28.

Waldo, who had served as New York City Fire Commissioner since the beginning of Mayor Gaynor's term in January 1910, was in office at the time of the Triangle Shirtwaist Factory fire which occurred on March 25, 1911 and killed 146 people.

On June 9, 1911, only 17 days after taking office as Police Commissioner, Waldo founded the motor-cycle squad, organized under the Office of Street Traffic Regulation Bureau.
Another of his early acts as Police Commissioner was the appointment of three "Strong Arm" anti-vice squads and their commanders—one of whom, Charles Becker, was later executed for complicity in the July 1912 murder of the bookmaker Herman Rosenthal (shortly after Rosenthal had told the press of extortion by Becker and other police).

Acting Mayor Ardolph Kline's elected successor, John Purroy Mitchel, indicated before taking office at noon on January 1, 1914, that he would not keep Commissioner Waldo in office.
Waldo tendered his resignation effective at midnight December 31, 1913, after transferring or accepting resignations from most of the department's senior officers and specialists.
Amid much confusion and discord, Mayor Kline (who had taken office after Mayor Gaynor's death in September 1913) refused to accept Waldo's resignation (which would have left the Department without leaders for the first 12 hours after New Year's Eve) and fired him instead.
He joined the Seventeenth Infantry Regiment of the United States Army as a Second Lieutenant in 1899, after the United States had occupied the Philippine Islands in the Spanish–American War. In the course of nearly four years in the Philippines, he served under General Arthur MacArthur, Jr., was on the staff of General Leonard Wood during the Moro rebellion, and commanded a battalion of Philippine Scouts. He resigned from the Army in 1905 with the rank of Captain, and became New York's First Deputy Commissioner of Police in January 1906, at the age of 28.
Waldo, who had served as New York City Fire Commissioner since the beginning of Mayor Gaynor's term in January 1910, was in office at the time of the Triangle Shirtwaist Factory fire which occurred on March 25, 1911 and killed 146 people.
On June 9, 1911, only 17 days after taking office as Police Commissioner, Waldo founded the motor-cycle squad, organized under the Office of Street Traffic Regulation Bureau. Another of his early acts as Police Commissioner was the appointment of three "Strong Arm" anti-vice squads and their commanders—one of whom, Charles Becker, was later executed for complicity in the July 1912 murder of the bookmaker Herman Rosenthal (shortly after Rosenthal had told the press of extortion by Becker and other police).
Acting Mayor Ardolph Kline's elected successor, John Purroy Mitchel, indicated before taking office at noon on January 1, 1914, that he would not keep Commissioner Waldo in office. Waldo tendered his resignation effective at midnight December 31, 1913, after transferring or accepting resignations from most of the department's senior officers and specialists. Amid much confusion and discord, Mayor Kline (who had taken office after Mayor Gaynor's death in September 1913) refused to accept Waldo's resignation (which would have left the Department without leaders for the first 12 hours after New Year's Eve) and fired him instead.
Rhinelander Waldo