Reddie Lang

J. J. Lang.

Originally a member of the 3rd L.R.V., Lang left that club and joined the Clydesdale in 1874. He played in the final, I think, as centre forward, and backed up Mr. J. R. Wilson. Possessing splendid dribbling powers, he was a very "showy" player, but his short steps did not make anything like the progress with the ball one imagined at the time. He was a somewhat heavy charger when he got the chance, and frequently preferred to take his man before the ball.
Scottish Football Reminiscences and Sketches by D. D. Bone 1890

James J 'Reddie' Lang was a Scottish forward who played in the 1870s and 1880s. He probably provides us with the earliest evidence of professionalism in association football.
Here are some milestones in Lang's career:

1874: Joined Clydesdale from Third Lanarkshire Rifle Volunteers.

 1874: On March 21st Lang played in the first Scottish Cup final. He was in the Clydesdale team that lost  2-0 to  Queen's Park. 

1876: Lang played in the Glasgow representative XI in their fixture against Sheffield on  February 19th. Glasgow won 2-0 and the Sheffield representatives were impressed with his display to the extent that they invited him to join The Wednesday.

1876: On March 25th Lang, still a Clydesdale player, made his international debut. He scored Scotland's 2nd goal in the 4-0 win over Wales.

1876: Lang joined The Wednesday and was given a job in the office of a blade manufacturer. He had no actual duties and stated that he used to just read the newspapers in the office. This effectively makes Lang the first paid professional in football history, although it would be another 9 years before professionalism was accepted in England and 17 in Scotland. 

1877: Lang returned to Scotland, joining Third Lanarkshire Rifle Volunteers.

1878: In his 2nd and last international Lang scored Scotland's final goal in a 9-0 rout of Wales (March 23rd).  

1878: The Scottish Cup Final-  Vale of Leven  1-Third Lanarkshire Rifle Volunteers 0. (March 30th).

1879: Lang rejoined The Wednesday. In the meantime he had lost the sight of an eye whilst working in the shipyards. He hid this minor detail from his new employers. 

1885Professionalism legalized in England.

1886: The Wednesday remained (officially) an amateur side so, ironically, Lang left the club who had paid for his services ten years before to pursue an openly professional career with Burnley.