Needing 180 to avoid an innings defeat, the hosts were 229 for two when the players shook hands at 4.50pm, Trescothick having contributed 129 not out and skipper Chris Rogers an undefeated 75.
The two veteran left-handers, with combined ages of 78 and the small matter of more than 48,000 first-class runs between them, defied everything Lancashire could throw at them in an unbroken partnership of 168.
In the end, the only winner was a flat, dry pitch, which did offer some turn on the final day, but never gave much assistance to the seamers. Somerset took nine points and Lancashire 11.
The day began encouragingly for Lancashire when Kyle Jarvis removed the off stump of nightwatchman Tim Groenewald with the total on 13.
Left-arm spinner Simon Kerrigan was introduced from the River End as early as the 16th over and he soon dismissed Tom Abell for 10, taking a return catch off a leading edge to make it 61 for two.
At that stage Somerset looked in for a real battle. But Trescothick rode his luck at times to reach his fifty off 83 balls and Rogers provided solid support as they set about saving the game.
By lunch the pair had taken the score to 97 for two.
Mindful of Somerset’s middle-order collapse the previous day, Lancashire would still have fancied their chances.
With a bit more luck it might have been a different story for the visiting bowlers. Twice in one James Anderson over early in the afternoon session, Trescothick edged boundaries between first and third slip, with no second slip posted.
The former England opener had not been at his best, but reached an invaluable hundred in the final over before tea, off 196 balls, with 12 fours.
Rogers had just moved his half-century, off 147 deliveries, with six boundaries, and both he and Trescothick used all their experience to combat Kerrigan, bowling into the rough outside their off stumps.
It made for fascinating cricket in the Taunton sunshine and by tea Somerset were out of danger on 178 for two. With the pressure receding, Rogers began to play more shots and for all their commendable efforts Lancashire resigned themselves to the inevitable.
A curtailed final session saw both batsmen relax, without giving their wickets away. They now boast a combined total of 133 first-class centuries.
Somerset director of cricket Matthew Maynard said: “Today we saw some real old-fashioned attritional county cricket with two very experienced batsman battling against a turning ball.
“Marcus and Chris played so well, but we mustn’t forget James Hildreth’s century in the first innings. I thought he batted superbly.
“It wasn’t an ideal pitch and we hope there will be more pace in the next one we prepare here, but it did make for some fascinating cricket at times.
“Again we have shown character as a team to pick up our third draw against a very good Lancashire side. Now we have to try and get ahead in a game so we can exert pressure on the opposition.”
Lancashire head coach Ashley Giles said: “I have to be happy with the way we have performed in our opening two games.
“We knew first division cricket would be tough and that has certainly been the case for all four days on the pitch here.
“The surfaces of the wickets at Taunton don’t tend to break up. When we arrived I was expecting us to field first, but it was a used pitch and we thought it might spin as the game progressed.
“There was some turn today, but it was always going to be difficult to bowl Somerset out twice. If we continue to play the way we have in the opening two matches we will be alright.”