1860 U. S. census: Dennysville, Washington County, Maine, USA.
1880 U. S. census: Ward 5, Detroit, Wayne County, Michigan, USA. Occupation: Lawyer.
1885 New Mexico, Territorial Census: Silver City, Grant County, New Mexico, USA. Occupation: Lawyer.
1900 U. S. census: Pinos Altos, Grant County, New Mexico, USA. Occupation: Lawyer.

Detroit City Directory Search:

Pre-1873 Lincoln, George D. NONE
1873-1874 Lincoln, George D. clerk, bds. 41 Washington ave.
1874-1874 Lincoln, George D. law student. bds. 64 Miami ave.
1875-1876 Lincoln, George D. law student, Wm P. Wells, bds. 41 Washington ave.
1876-1877 Lincoln, George D. lawyer, Butler Blk, h. 207 Park.
1877 Lincoln, George D. lawyer, Butler Blk, h. 207 Park.
1878 Lincoln, George D. lawyer, Butler Blk, h. 140 High w.
1879 Lincoln, George D. lawyer, Butler Blk, h. 140 High w.
1880 Lincoln, George D. lawyer 84 Griswold, h. 457 2d ave.
1881 Lincoln, George D. lawyer 84 Griswold, h. 21 Ledyard.
1882 Lincoln, George D. lawyer 84 Griswold, h. 21 Ledyard.
1883 - forward Lincoln, George D. NONE

University of Michigan Law School was founded in 1859 and is the oldest Law School in Michigan, but on checking at this website, http://www.law.umich.edu/historyandtraditions/students/Pages/AlphabeticalListofGraduates.aspx, he did not attend there.

Per Mike McArthur, a Reference Librarian at the University of Michigan Law Library, S-136 Legal Research, (734) 763-5839:

First, I checked what is known as the Necrology Database. It is a database that includes all the information for any student that attended the University of Michigan (graduate or undergraduate) who died prior to 1965. I was unable to locate anyone named George Lincoln. (I included the link to database so you can poke around a bit with any extra info you may have.)

Next, I used some of the information on your information request to track down some additional information in the Detroit Directories from the mid-1800's (with targeted Google searches). 64 Miami Avenue was the address for the law firm Wilcox & Chase. The partners were George A. Wilcox and George N. Chase. Unfortunately I don't know anything more than that. I did not have time to do much with the Washington Ave. address.

I then checked to see if there were any law schools in Detroit at the time. I knew that Michigan State University College of Law "bought" the Detroit College of Law in 1995. The Detroit College of Law was the oldest law school in Detroit and was founded in 1891.

Going off of those 3 factors, I would guess that your relative was apprenticed for the 2-3 years before taking the "bar exam" and beginning practice in 1876-77.

So what is the apprenticeship process? I have copied two paragraphs from a well known article on the transition to a formal university run law school program. The article is called "Reconstructing Langdell" and can be found at 32 Ga. L. Rev. 1 (1997-1998).

"Before Langdell undertook to revolutionize the study of law, three overlapping paths to law study were available: (1) self study, (2) apprenticeship training under an experienced lawyer, and (3) law study within a freestanding law school or at a university. Although many well-known lawyers pursued self study, as did Abraham Lincoln, the path offered the new lawyer no guidance.

Absent the availability of a more experienced lawyer for consultation, the self-taught lawyer learned at the risk of his clients. The apprentice system offered more guidance, but it bore a cost. The law student worked in the office of an attorney and paid the attorney a fee for the privilege of reading the books in the lawyer's library.' But often, the lawyers practice did not afford the student a sufficiently broad library of books, and the apprentice still often studied unsupervised, gleaning whatever legal theory he could on his own. Sometimes young lawyers were merely used as free labor and relegated to menial work. Finally, many practitioners lacked both the time and the economic incentive to train young lawyers broadly in skills beyond those needed for the particular case at hand."