He graduated from Columbia College in 1824, studied law in New York City, and attained admission to the bar in 1828.

Once the conflict began, Duer supported the Union, but opposed the Republican Party and continued to advocate for concessions on the slavery and secession issues a way to end the war.

[2] Later that year, Duer and Elijah Paine Jr. published a reference work in octavo format, Practice of the Courts of Common Law in the State of New York.

[1] In Congress, Duer pursued a moderate course on slavery and was willing to allow it to continue where it already existed, but not to expand to new territories unless approved by the voters.

[5] During a discussion about the Speakership election, Duer stated that Richard Kidder Meade, a Virginia Democrat, was a dis-unionist who favored secession.

[6] A near-riot ensued, with Democrats attacking Whig and Free Soil Party legislators, and the Sergeant at Arms unable to immediately restore order.

[1] In 1854, Duer began to reside in San Francisco, California[1] and founded a law firm in partnership with Balie Peyton, Delos Lake, and Julius K.

[10] During the 1860 presidential election, Duer was opposed to secession and civil war, so he attempted to rally support for a moderate course that made concessions on slavery in order to prevent the slave holding states from leaving the Union.

[12][13] Later that year, Duer sent a widely reprinted letter to several newspapers in which he urged Bell and Everett supporters to switch their allegiance to Democrat Stephen A. Douglas as the best chance of preserving the Union.

[14] In the general election, Republican William E. Lansing soundly defeated Douglas Democrat D. Franklin Chapman.

[11] Among his proposals was one calling for the slave holding states to return to the Union so they could advocate for secession by means of a Constitutional amendment; in his view, if the southern states were determined to leave the Union, they were required to follow a legal process for doing so rather than engaging in armed rebellion.