In 1861 O'Donnell ran off to Missouri and enlisted in the Union Army at the start of the CIVIL WAR. He was with Mulligan's Brigade when it was captured by Price at Lexington, Missouri, September 20, 1861. O'Donnell was paroled and returned home.
He read law with Slagle & Acheson of Fairfield and in about 1866 moved to Dubuque and entered practice as a member of the firm of Wilson, Mulkern & O'Donnell. Following the death of his partners, O'Donnell formed a partnership with his brother, Hubert, and Judge McCeney. (3) In 1869 he was elected representative, was re-elected in 1871, and elected again in 1879, serving in the Thirteenth, Fourteenth and Seventeenth general assemblies.
In 1882 he was elected mayor of Dubuque, and was re-elected in 1883. He was elected a judge of the District Court and and served for sixteen years from 1891 to 1906. He died as a result of an automobile accident.
Judge O'Donnell was once married to Fannie STOUT. (4)
1. "Pay Tribute to Judge O'Donnell," Telegraph Herald, Sept. 10 1925, p. 4. Online: http://news.google.com/newspapers?id=GiBFAAAAIBAJ&sjid=OLsMAAAAIBAJ&pg=1690,2195025&dq=frederick+o-donnell+dubuque&hl=en
3. "Judge O'Donnell Dies of Injuries," Telegraph Herald, September 9, 1925, p. 2. http://news.google.com/newspapers?id=GSBFAAAAIBAJ&sjid=OLsMAAAAIBAJ&pg=4682,2047816&dq=frederick+o-donnell+dubuque&hl=en
4. "O'Donnell Will Filed in Court," Telegraph Herald, September 30, 1925, p. 2. Online: http://news.google.com/newspapers?id=KyBFAAAAIBAJ&sjid=OLsMAAAAIBAJ&pg=5388,4731358&dq=frederick+o-donnell+dubuque&hl=en
The Iowa Legislature, "Frederick O'Donnell," Online: https://www.legis.iowa.gov/Legislators/legislator.aspx?GA=13&PID=4730