Breaking a 16-year Republican hold on the office Mr Patrick defeated Republican Kerry Healey, according to early projections by the CBS and ABC news affiliates in Boston.
Analysts say Ms Healey, who would have been the state's first female elected governor, lacked Mr Patrick's charisma while portraying him as a classic tax-and-spend liberal who is soft on crime.
Mr Patrick exploited Ms Healey's links to Republican Governor Mitt Romney, whose popularity has sunk in the liberal state as he carves out conservative stances on hot-button policy issues to rally conservative Republicans ahead of a potential presidential bid in 2008.
Mr Romney decided not to seek another term as governor.
Mr Patrick, 50, a Harvard-educated lawyer who served as assistant US attorney general for civil rights under President Bill Clinton and held executive jobs at Texaco Inc. and Coca-Cola Co., rallied Democratic faithful in a grass-roots campaign.
He also won over moderate Republicans and independent voters impressed by the clean-cut corporate lawyer who rose from poverty on Chicago's South Side.
Mr Patrick's campaign, focused on "the politics of hope," called for expanding health care, improving public schools, overhauling creaking infrastructure and attracting growth industries to create jobs, in the only US state whose population has declined for two straight years.
"It marks the emergence in Massachusetts of a new kind of Democratic moment," said Julian Zelizer, Boston University professor of contemporary American politics.
"Given the fact that he is in a national spotlight, I think it's going to help put Massachusetts at the forefront of a national debate about the Democratic Party," he said.
Mr Patrick's rise underscores dramatic social change in a state where black school children were pelted with rocks and bottles as they were bused into Boston's white neighbourhoods 32 years ago in court-ordered school desegregation.
Six African Americans are making serious bids for senator or governor positions across the country.
Historically, African Americans have faced steep obstacles running for state-wide offices in the United States. Only one African American, Democrat Douglas Wilder of Virginia, has been elected governor, holding the job from 1990 to 1994.