In 2287, J'onn was recruited by Sybok and served in the Galactic Army of Light. He, along with Sybok and his numerous followers, stormed Paradise City on Nimbus III. He warned St. John Talbot, the Federation's representative, to get away from the city's transmitter when he tried to call for assistance.
Afterwards, he accompanied Sybok on his voyage to the center of the galaxy on the USS Enterprise-A, the vessel dispatched by Starfleet to rescue the hostages on Nimbus III. After the shuttlecraft Galileo docked forcefully on the Enterprise, he escorted Captain James T. Kirk, Captain Spock, and Dr. Leonard McCoy to the ship's brig on Sybok's orders. Later, he was injured on the bridge, when Klaa attacked the Enterprise. He later served beverages to both the Enterprise-A crew and the Klingon contingent of Klaa's ship, in the Enterprise's observation lounge. (Star Trek V: The Final Frontier)
Background information Edit
In the revised final draft of the script for Star Trek V, J'Onn is introduced as "a ragged and malnourished homesteader of some alien race." 
J'onn was played by actor Rex Holman, who won the role after auditioning for the part. William Shatner commented, "His face was so wonderfully lean, so gaunt, that it lent itself perfectly to the image we had of J'onn." (Captain's Log: William Shatner's Personal Account of the Making of Star Trek V: The Final Frontier, p. 78)
Recalling the prosthetics for the part, Kenny Myers – who designed Star Trek V's special make-up effects – stated, "Jim Cale made a brow piece to cover Holman's eyebrows, and I made him a set of really stunted teeth that almost looked like they'd been gnawed down to the gum, which made him look really pathetic. I designed the character but it was [makeup artists] Wes and Jeff Dawn who applied the make-up on the set each day." (The Making of the Trek Films, 3rd ed., p. 87)