The veteran vessel has been serving the Ardrossan to Arran route since the MV Caledonian Isles went in for an overhaul, which has now taken a year to complete.
The Isle of Arran has now moved up the coast to Oban to support services to Mull for the coming weeks.
MV Glen Sannox operated three return sailings to Arran when it officially entered service on Monday, January 13, carrying over 1,000 passengers between Troon and Brodick over the course of the day.
CalMac’s CEO Duncan Mackison said: “MV Glen Sannox entry into service was an historic day for Arran, as she successfully operated all scheduled sailings on a day when many other routes and vessels were disrupted by weather conditions.
“In fact, she was able to carry all traffic booked to Arran which was displaced by other cancellations.
“We hope this is a sign of things to come for Arran in terms of improved reliability and capacity and is something other routes will benefit from as more new vessels join the fleet in the coming months.
“We have long talked about a new vessel benefiting more than the route she directly serves. Now we are going to see that in action later this week when MV Isle of Arran moves to Mull and a two-vessel service is reinstated. The vessel will also cover some services to Colonsay, Coll and Tiree.”
MV Alfred enters annual overhaul on Monday, February 10, so deployment plans for routes covered by major vessels from then on will remain under review.
Ardrossan will be without a ferry service for two months, until the MV Caledonian Isles returns to the harbour after its long time away.
The new Glen Sannox will operate from Troon in the meantime, as the design makes it unable to berth at Ardrossan until a harbour redevelopment is finalised and complete.
Negotiations over who pays what for the upgrade are continuing.