Law school being a non-existent concept as well in the mid-19th century, Frelinghuysen became a lawyer after an apprenticeship with his uncle/adoptive father Theodore, whose practice he inherited in 1839 at age twenty-two.
After becoming a successful attorney representing the biggest businesses of the day, including the Jersey Central Railroad and the Morris Canal & Banking Company. Frelinghuysen became active in politics, getting a seat on the Newark Common Council and becoming city prosecutor.
Frederick Frelinghuysen was always anti-slavery, but he hesitated about joining the new Republican Party.
Propelled by his powerful family and own abilities, Frelinghuysen's big political break came in 1861. In early 1861 Frederick Frelinghuysen was the New Jersey delegate to the Washington Peace Conference (aka, the "Old Gentleman's Convention").
The conference was of course a failure, but Frederick continued his political rise. In 1861 Frelinghuysen took over his uncle's old post as New Jersey attorney General. In 1866 Governor Marcus Ward appointed to fill an unexpired term in the United States Senate. In the Senate, Frelinghuysen joined the Radical Republicans, favoring rights for blacks and voting to impeach and remove from office President Andrew Johnson.
By 1869 the Democrats were back in charge of the New Jersey legislature and Frelinghuysen was not reelected to the United States Senate. President Grant offered to make Frelinghuysen Ambassador to Great Britain, but Frelinghuysen declined the post, choosing to stay in New Jersey so that he could eventually return to the Senate.