Ohnimus was born in the “south of the slot” neighborhood of San Francisco on June 3, 1893. He was the son of Louis Juan Ohnimus, who was the Superintendent of Woodward Gardens amusement park in San Francisco (circa the 1880s) and later President of the San Francisco Civil Service Commission in 1902. Arthur’s mother was Grace D. Pierce, who was a famous San Francisco stage actress who was instrumental in the mayoral campaign of E.E. Schmitz in 1902.
Arthur Ohnimus was raised in the Western Addition neighborhood and attended Hearst Grammar School and Sacred Heart College. At age 21, Arthur began his long career at the State Capitol when he was appointed as a committee clerk in the California Assembly in January 1915. During his time as a clerk in the Assembly, Ohnimus worked alongside fellow young aspiring clerks such as Earl Warren (later U.S. Chief Justice) and Arthur Samish (later a legendary lobbyist). Arthur’s first known exploration into partisan politics was in 1916 when he campaigned for Progressive San Francisco Assembly Member Nick J. Prendergast. In subsequent years, he would also manage the campaign of S.F. District Attorney Matthew Brady. Ohnimus later abandoned partisan politics in favor of nonpartisan clerk positions in the state Assembly.
In 1921, Ohnimus was elected Minute Clerk of the Assembly. It was also in June 1921 that he graduated with a law degree from St. Ignatius College (St. Ignatius would later be renamed University of San Francisco). He was President of the Native Sons of the Golden West “El Dorado Parlor” in San Francisco in the 1920s. He was admitted to the State Bar in March 1922. On January 8, 1923, Ohnimus was elected Chief Clerk of the Assembly for the Forty-Fifth Session. He was reelected each session thereafter until his retirement in 1963 (with the exception of 1937-1940 when he did not seek election). In 1957, the Assembly Rules Committee appointed Ohnimus as the Assembly’s first Chief Administrative Officer, making him the first full-time Assembly employee. During his years as Chief Clerk, Ohnimus oversaw the installation of the electronic voting system and public address system, and the evolution of the legislature from a weak, part-time body, to a stronger, more active “hybrid” institution, with career staff and more complex legislation.
Ohnimus married Bernice M. Wemple on December 27, 1943. Bernice was the daughter of Judge N.V. Wemple, who served in the California Assembly from 1924 to 1928, representing Lassen County. Arthur Ohnimus died on March 13, 1965, at the age of 71. He is buried at East Lawn Cemetery in Sacramento, California.