Doctor Lucas joined the Interspecies Medical Exchange where he met and befriended fellow doctor Phlox. Eventually, the two got different postings, but they remained in regular contact through letters.
In 2151, he got a position in a busy city hospital on Denobula, and in one of his letters to Phlox he complained about the stress of the mating season, but found the Denobulan people amazingly complex. (ENT episode: "Dear Doctor")
In early 2154, he was assigned to the IME pathogen storage and research facility Cold Station 12 as Senior Medical Director. A crisis soon arose when Doctor Arik Soong and his Augments attacked the station to steal the Augment embryos stored there. Lucas refused to give Soong the access codes even when Deputy Director Iyer was killed with Symbalene blood burn. He remained defiant until Malik threatened to kill Phlox as well, who had arrived on the station as part of a rescue team from the Enterprise (NX-01). The Augments fled with the embryos, but before he left, Malik flooded most of C12 with the stored pathogens. Lucas and his staff remained behind for clean up efforts that he estimated would take a year. (ENT episodes: "Cold Station 12", "The Augments")
When a plague broke out on the planet Sauria in 2164, Lucas was member of the IME relief team that was sent there. For months, Lucas and his fellow doctors worked hard to find a cure, but they were unsuccessful until Charles Tucker III, disguised as Albert Sims, and Antonio Ruiz managed to obtain an antiserum from on of Maltuvis's hospitals. It turned out that the Saurian dictator used the plague as a means to invade one country after another on the planet and turn the population against off-worlders. Lucas and Ruiz gave the cure to Presider Moxat of the Global League, who distributed it as a Saurian invention. (ENT - Rise of the Federation novel: Tower of Babel)
The next year, Lucas returned to Denobula to attend the wedding of Vaneel, one of Phlox's daughters and Pehle Retab, an Antaran. After the ceremony, the father-in-law, Sohon Retab was murdered by Mettus and Phlox had to remain behind for the trial and to support his family. Since Phlox's ship the USS Endeavour had to depart for Ware space for an important mission, Doctor Lucas agreed to to take his friends place as chief medical officer temporarily. (ENT - Rise of the Federation novel: Live by the Code)