John Dahl was admitted to the bar after graduating the University of Minnesota College of Law in 1892. After a year studying music in Europe with his wife, he returned to Minneapolis and was appointed court reporter for Seagrave Smith of the Hennepin County District Court. Upon Smith's death in May 1898, he became reporter for Judge Alexander M. Harrison, serving until 1905, when he was appointed an Assistant Hennepin County Attorney. He quickly gained a reputation as an excellent trial lawyer, as recalled by a friend:
"His ability and success, particularly as a trial lawyer, was very generally recognized, and I believe it may be fairly said that he will long be remembered as one of the truly great trial lawyers of this section of our country."
In 1910, he became employed by the Minneapolis Street Railway Company, and represented it in many cases in court for eight years. After the death of his wife in 1926, he moved to California, practiced awhile with a son, and returned to Minneapolis in 1933, where he resumed private practice. He died on May 6, 1938, at age sixty-eight.
One of Dahl's favorite stories involved his transcription of Judge Seagrave Smith's jury instructions in his last case in April 1898. After the jury returned a defense verdict, an appeal was taken to the Minnesota Supreme Court. Its decision in Rossman v. Moffett, 75 Minn. 289, 77 N.W. 960 (1899) is posted in the Appendix.