Howlett was born in Chicago, a son of Irish immigrants. As a youth, Howlett was All-American water polo player, participating on ten championship teams of the Illinois Athletic Club. He graduated from St. Mel High School and briefly attended DePaul University, leaving the latter in 1934 to become a state bank examiner. Subsequently, he founded his own insurance business, served as Chicago-area director of the National Youth Administration, was an executive for the Chicago Park District, was appointed regional director of the U.S. Office of Price Stabilization, and was a steel company executive. He was also a U.S. Navy veteran of World War II.
In 1956, Howlett ran for Illinois Auditor and is credited with exposing embezzlement by incumbent Auditor Orville Hodge of more than $1.5 million in state funds. Hodge resigned and eventually went to prison, but Howlett lost the general election to Elbert Sidney Smith as part of a national Republican landslide. However, in the next general election, in 1960, Howlett was elected Auditor of Public Accounts (the Auditor's Office was the predecessor to the current office of Comptroller), and was re-elected twice. During Howlett's first term as Auditor, he cut the budget of the office by one-fifth, and returned over $600,000 to the state treasury. In 1972, Howlett was elected Illinois Secretary of State, becoming the first Democrat state officer to win four consecutive statewide elections.
Howlett was prepared to run for re-election in 1976, but was encouraged by Cook County Democrats to challenge incumbent Governor Dan Walker for the Democratic nomination in 1976. Howlett defeated Walker in the March primary, and stood as the Democratic nominee for governor of Illinois in the general election, whereupon he was defeated by Republican nominee James R. Thompson.
Throughout the campaign, Howlett was dogged by conflict of interest charges, first raised by Walker, over payments Howlett received as an executive at Sun Steel Company. A report issued by former Illinois Supreme Court Justice Marvin Burt at the behest of Republican state Attorney General William J. Scott was highly critical of Howlett. However, a Cook County judge ruled no conflict of interest had arisen, and cleared Howlett. Thompson, who successfully prosecuted former Illinois governor Otto Kerner, Jr., continued to hammer the issue during the general election campaign, and attacked Howlett as corrupt, and Attorney General Scott vowed to appeal the judge's ruling. Ironically, it was Scott who later was forced to resign after a felony conviction.
Early polls of the contest had Howlett in the lead, although Thompson had nearly closed the gap by the time of the primary. However, Walker's attacks during the bitter primary weakened Howlett, and by August, Thompson held a slim lead in the polls. His lead expanded during the campaign, and Howlett ended up losing by 30 percentage points (nearly 1.4 million votes), the widest margin of defeat for any Democratic Nominee for Governor of Illinois in history. Thompson was the first candidate for Governor to receive over 3 million votes, and his tally of 3,000,395 remains the highest number of votes ever cast for a candidate in an election for Governor of Illinois.
After his loss in the 1976 governor's race, Howlett opened a private consulting business.
Howlett would later see his son run for statewide office through bizarre circumstances. In the 1986 Democratic primary for Lieutenant Governor, former U.S. Senator Adlai Stevenson III and the Democratic Party selected State senator George E. Sangmeister as the party-preferred candidate, however he narrowly lost the primary to Mark Fairchild (a Lyndon LaRouche activist). After LaRouche followers had won the Democratic nominations for both Lieutenant Governor and Secretary of State, Stevenson refused to run as the Democratic standard-bearer, and formed the Solidarity Party. When Sangmeister was unwilling to run with Stevenson in the fall, Howlett's son Michael J. Howlett, Jr., then a Cook County judge, was nominated by the Solidarity Party. Stevenson-Howlett went down to defeat in the fall, with only 40% of the vote. Another son, Edward G. Howlett, was the unsuccessful Republican nominee for Chicago City Clerk in 1995.
Howlett died in Chicago's Mercy Hospital of chronic kidney failure. He had suffered a stroke three months earlier and remained hospitalized from then until his death.
The building housing the offices of the Illinois Secretary of State in Springfield, Illinois, formerly known as the Centennial Building, is named after Michael J.Howlett.