He married Mary Louise Duff, September 26, 1867.
She was the daughter of Peter Duff (February 16, 1802 – September 13, 1869), the founder of  Duff's Business Institute in Pittsburgh, (now Everest Institute).

John and Mary Dalzell had five children: William Sage Dalzell (August 17, 1868 – September 27, 1934), Elizabeth Marter Dalzell (March 4, 1870 – 1961), Samuel Dalzell (April 22, 1873 – 1958), John Dalzell Jr (August 1, 1875 – May 6, 1877) and Robert Duff Dalzell (August 27, 1882 – November 30, 1967).

Elizabeth Marter Dalzell married Col.
George M. Dunn,  one of Teddy Roosevelt's Rough Riders.

During John Dalzell's political career, his wife Mary Louise was a prominent hostess active in the Washington political scene.
Although women did not have the vote at that time, still she was politically influential.
To briefly quote a passage describing this from vol 2, p 25 of Kitzmiller's The Family Encyclopedia:
"... his friends persuaded him, somewhat against his will, to go to Congress.
In this project they were aided and abetted by his wife .... (who) was exceptionally well-fitted to be the helpmate of a congressman.
Relieving her husband of many of the burdens which constituents place upon the representative, she also unobtrusively engineered campaign contracts, quietly supervising that meticulous reelection detail which is the bane of a congressman's existence.... The death of Mrs Dalzell in 1909 broke the mainspring of his political existence."

He retired in Washington, D.C., following the death of his wife in 1909.
In 1925, he moved to live in the home of his youngest son, Samuel Dalzell, in Altadena, California.
He died in California at the age of 82.
 He is buried in Pittsburgh's Allegheny Cemetery.
He married Mary Louise Duff, September 26, 1867. She was the daughter of Peter Duff (February 16, 1802 – September 13, 1869), the founder of  Duff's Business Institute in Pittsburgh, (now Everest Institute).
John and Mary Dalzell had five children: William Sage Dalzell (August 17, 1868 – September 27, 1934), Elizabeth Marter Dalzell (March 4, 1870 – 1961), Samuel Dalzell (April 22, 1873 – 1958), John Dalzell Jr (August 1, 1875 – May 6, 1877) and Robert Duff Dalzell (August 27, 1882 – November 30, 1967).
Elizabeth Marter Dalzell married Col. George M. Dunn,  one of Teddy Roosevelt's Rough Riders.
During John Dalzell's political career, his wife Mary Louise was a prominent hostess active in the Washington political scene. Although women did not have the vote at that time, still she was politically influential. To briefly quote a passage describing this from vol 2, p 25 of Kitzmiller's The Family Encyclopedia:
"... his friends persuaded him, somewhat against his will, to go to Congress. In this project they were aided and abetted by his wife .... (who) was exceptionally well-fitted to be the helpmate of a congressman. Relieving her husband of many of the burdens which constituents place upon the representative, she also unobtrusively engineered campaign contracts, quietly supervising that meticulous reelection detail which is the bane of a congressman's existence.... The death of Mrs Dalzell in 1909 broke the mainspring of his political existence."
He retired in Washington, D.C., following the death of his wife in 1909. In 1925, he moved to live in the home of his youngest son, Samuel Dalzell, in Altadena, California. He died in California at the age of 82.  He is buried in Pittsburgh's Allegheny Cemetery.
John Dalzell