The junior Justice, defined as the newest member to the ranks of Associates, on the USSC is required by tradition to perform menial tasks like open the door during conference sessions and serve on the Cafeteria Committee. Gorsuch, assuming a new judge is in place in the historical average of about 2 months, had one of the shortest tenures, about a year. Who had the longest?
Looking through the list of judicial appointments at Wikipedia, I think that this was Justice Joseph Story. He was appointed in February 1812 by James Madison. No further justices were appointed until Justice Smith Thompson in Sept. 1823, a period of 11½ years.
In more recent times Justice Stephen Breyer was appointed in August 1994, and there were no further appointments until Justice John Roberts was appointed in Sept 2005, 11 years later. However John Roberts was appointed as Chief Justice, so the next associate Justice was Samuel Alito in January 2006, 11¼ years later, so Justice Story still holds the title.
In proportion to the time they spent as Supreme Court Justices, Arthur Goldberg and Robert Trimble were the Junior Justice for their entire Supreme Court careers (Trimble died in office, and Goldberg resigned to become ambassador to the UN).