Lovejoy attended both Cambridge College and Amherst College. Then he studied law and was admitted to the bar.
At sixteen Lovejoy began to work with a Boston mercantile house. Later he moved to Sparta, Missouri (1840) and began the practice of law. In the spring of 1842 he joined Dr. Elijah White, United States Indian agent, on his way to Oregon, and was one of three men who recorded the experiences and discoveries of the trip. Dr. Marcus Whitman, head of the mission at Waiilatpu, where the party stopped in October 1842, persuaded Lovejoy to return with him to the states. Together the two men started on their arduous ride in November. Whitman finally reached Missouri in February by the southern route through Santa Fé, Mexico. Lovejoy stayed at Bent's Fort on the headwaters of the Arkansas River until the following July, when he joined Whitman and the emigrant train and returned to Oregon, settling at Oregon City.
Lovejoy began the practice of law but took an active part in public affairs. In 1844 he was chosen one of the eight members of the legislative committee of the provisional government, which remodeled the organic laws drawn up the year before. During the same year he held the office of attorney-general. In 1845 he was defeated as candidate for governor against George Abernethy but was elected mayor of Oregon City. Running against Abernethy again in 1847, he lost the election by only sixteen votes. In 1846 and again in 1848 he was sent to the provisional legislature. After the Whitman massacre, Lovejoy together with Jesse Applegate and Governor Abernethy pledged their personal credit to the Hudson's Bay Company for supplies to carry on war against the Indians. Lovejoy served as adjutant-general during the war. He was elected supreme judge in 1848 and held that position until the establishment of the territorial government in 1849. He was speaker of the House in the first territorial legislature, a member of the council in 1851-1852, postal agent in 1853, and delegate to the lower house in 1854 and again in 1856.
Lovejoy was a stockholder in the People's Transportation Company, which was organized in 1862, to promote a project for a portage at Oregon City. He was also a member of the Oregon City Woolen Manufacturing Company, which was incorporated in 1863, and a director of the Oregon Telegraph Company. He was interested in railroads and in 1847 acted as chairman of a public meeting called to memorialize Congress on the construction of a transcontinental line. Later he became one of the directors of the east-side Oregon Central Railway Company, which completed the first railroad through the Willamette Valley. In 1873 he moved to Portland where he remained until his death.
Lovejoy was a member of the Democratic Party.
Lovejoy married Elizabeth McGary in 1845.