On election day, Reagan won 51.22% of the vote in the state to Mondale's 48.43%, a margin of 2.79%.
Massachusetts had been a Democratic-leaning state since 1928, and a Democratic stronghold since 1960. In 1972, Massachusetts was the only state in the nation to vote for Democrat George McGovern over Republican Richard Nixon in the latter's 49-state landslide. However in 1980, Reagan had managed to win the state for the GOP for the first time since 1956 in a 3-way race with a plurality of only 41.90% and a razor-thin margin of 0.15%. Thus in a 1984 head-to-head match-up, Massachusetts was one of the few states whose outcome remained in doubt as Reagan appeared poised for a convincing win nationwide.
Ultimately, in the midst of a decisive nationwide Republican landslide, Reagan would narrowly triumph in Massachusetts, as he did in 48 other states, leaving Mondale to win only his home state of Minnesota and the District of Columbia. Reagan's win was the first time a Republican had won an absolute majority of the popular vote in Massachusetts since 1956, although it was still Reagan's narrowest win in the nation, thus making it the second most Democratic state after Minnesota. Massachusetts was about 16% more Democratic than the national average in the 1984 election.
Reagan carried 9 counties in Massachusetts to Mondale's 5. Reagan's strongest county was suburban Plymouth County, where he took 60.2% of the vote. Mondale's strongest county win was Suffolk County, home to the state's capital and largest city, Boston, where he took 62.3% of the vote.
1984 remains the last time to date that a Republican presidential candidate has carried Massachusetts.