Born near Stanford, Kentucky, on August 4, 1818, Rousseau attended the common schools as a child. His father, David Rousseau, brought his family across the Appalachians from Virginia, but he had a difficult time regaining economic equilibrium. Lovell's elder brother had already left home, so when their father died of cholera attempting to move the family to a new home in 1833, it fell to Lovell and his younger brothers to dig their father's roadside grave. At age fifteen, he had become his family's primary breadwinner. Soon afterwards, he was forced to sell his family's slaves in an effort to cover the family's debts. Eager to earn a wage, he began working on a road-building crew, traveling around the Midwest. Determined to rise, he studied grammar, mathematics, and French, and returned to Kentucky where he read law in Louisville, Kentucky, for several months. In 1841, he passed the Indiana bar examination and began practicing law with his brother, Richard Hillaire Rousseau, as junior partners in a firm led by James I. Dozier, in Bloomfield, Indiana. Both brothers married Dozier's daughters. Richard married Mary E. Dozier in 1839, while Lovell married Maria A. Dozier in 1843. Lovell successfully ran for the Indiana House of Representatives as a Whig candidate in 1844, and in 1846 he was commissioned as a captain in the Mexican–American War and charged with raising a company of volunteers. He led them at the Battle of Buena Vista, where he helped rally the Indiana troops at a key point in the battle. When he returned from the war, he gained a seat in the Indiana Senate and continued to run a successful law practice. After relocating to Louisville, Kentucky, he served in the Kentucky Senate from 1860 to 1861.
Rousseau was elected an Unconditional Unionist to the United States House of Representatives in 1864, serving from 1865 to 1866. As a former military officer, he served on the Committee on Military Affairs. In June 1866, relations between Rousseau and Iowa congressman Josiah Bushnell Grinnell became tense. The two had a series of debates over a bill intended to give more power to the Freedman's Bureau. Rousseau opposed it having seen and heard about rebellious and illegal actions by agents working for the bureau, whereas Grinnell strongly supported the bill as a former active abolitionist and aide to runaway slaves. The debates eventually turned into mudslinging, Grinnell questioning Rousseau's military record and insulting his performance in battle as well as a few comments on the state of Kentucky. On June 14, 1866, Rousseau approached Grinnell in the east portico of the capitol building after a session of congress. He told Grinnell that he had been waiting for an apology from him for the insults he made about him before the House. Grinnell pretended not to know what Rousseau was talking about, enraging Rousseau who struck him repeatedly with the iron handle of his rattan cane until it broke. He struck him chiefly in the face but a few blows hit Grinnell's hand and shoulder. Grinnell walked away with only bruises and did not have to be absent from congress at all. However, a committee was organized to investigate the incident which was composed of Nathaniel P. Banks, Henry J. Raymond, Rufus P. Spalding, M. Russell Thayer and John Hogan. Rousseau was reprimanded for his actions and soon later resigned. He was elected back the same year to fill the vacancy caused by himself and continued to serve until 1867.