Benjamin, making his third appearance on loan from Leicester, took only 12 minutes to make a mockery of Plymouth's club record seven consecutive clean sheets.
The robust former Cambridge United forward turned his marker from a Richard Carpenter header to fire his first goal for the club via defender Paul Connolly.
Knight added his 20th of the season and 19th in the league on 34 minutes, sweeping in Adam Virgo's cross with a right-foot shot from ten yards.
Plymouth, outplayed in the first half, fought back after the break following a double substitution by manager Paul Sturrock.
He introduced Marino Keith to form a three-pronged attack and Brighton keeper Ben Roberts was forced into a flying save from a David Friio header.
The home side were disrupted by an injury to John Piercy and Argyle finally pulled a goal back with two minutes left.
A free-kick from deep flicked off the head of Brighton captain Danny Cullip back towards his own goal.
Roberts made a sprawling save at the foot of his post, but Nathan Lowndes tucked in the rebound from close range.
It came too late to save Argyle, but they are still five points clear at the top after a surprise defeat for QPR at lowly Chesterfield.
Brighton manager Mark McGhee said: "That is the best performance by my side since I came here. We played at a good tempo and deserved our victory.
"I was delighted for Trevor Benjamin - he looked a better player than this division."