He appeared in the first Monkey Island game. He didn't believe the stories of LeChuck. He believed Guybrush Threepwood to have the silliest name he had ever heard (although his own is not much better). When asked about LeChuck, he would point to Estevan who seemed to be the resident expert.
In the DemoEdit
Even though he appears in the SCUMM Bar, Guybrush refuses to talk to him, saying "I don't want to come between a pirate and his grog."
Mancomb's name seems to be a pun on Guybrush (man/guy and comb/brush) while "Seepgood" rhymes with "Threepwood". This is especially humorous considering that he reveals his name right after laughing at Guybrush's name.
In Escape from Monkey Island when Guybrush returns to the mansion after finding the Ultimate Insult, when he sees LeChuck at the desk and says "LeChuck" he retorts "That's GOVERNOR LeChuck to you, Seepgood!", making a reference to Mancomb Seepgood.
In the Wicher 3: Blood and Wine DLC you can insult fistfight with a character called Mancomb.