Howarth was born on a farm in Green County, Indiana and spent his early childhood on a farm in Clark County, Illinois. He moved with his parents to Harvey, Illinois when he was eight. He was a straight-A pupil in school. After working to save a nest-egg of $750, he entered the University of Illinois, where he received his law degree in 1934. He met his wife Mary while they both were students at the U. of I. He came to Springfield in 1934 as a title attorney for the Prudential Insurance Company.
During World War II he was an officer aboard the aircraft carrier U.S.S. Lexington in the Pacific. He earned six battle stars and a presidential unit citation.
Howarth worked in George Coutrakon's campaign for State's Attorney of Sangamon County, then served under Coutrakon as Assistant State's Attorney in 1948-49. He and State's Attorney Coutrakon made determined (and largely successful) efforts to eliminate gambling and prostitution in Sangamon County, along with the corruption they caused in city and county governments.
As mayor he was a champion of civil rights and impartial law enforcement. In his first term he promptly began to desegregate the police department, the fire department, and the municipal band and choir. He informed Springfield restaurant owners who refused to serve blacks that they were in danger of losing their liquor licenses.
Howarth urged the annexation of the burgeoning subdivisions surrounding Springfield, adding 10,000 acres to the city during his three terms. Under his leadership, the city expanded Capital Airport, built the Prairie Capital Convention Center, and added Riverside Park and Lincoln Greens Golf Course to the park system. He also played a major role in the development of the Old Capitol Plaza, the establishment of Sangamon State University, and the creation of the Lincoln Home National Historic Site.
He persuaded the City Council to create Commissions for Human Relations, City Planning, Historic Site Development, and Urban Renewal. During his final term as mayor Springfield won the All-American City Award.
The late Dr. Edwin Lee, a prominent member of Springfield's black community, is quoted in Howarth's obituary in the State Journal-Register (October 17, 1991): "Civil rights made the greatest strides in Springfield since Lincoln's time under his regime. What he did was unheard of. It lifted the hearts and souls of many people in the so-called minority community."
Mayor Ossie Langfelder is quoted: "He was, in my estimation, probably the most outstanding mayor Springfield ever had."