Saints were always in control, despite leading by only six points at half-time, but Salford gave a good account of themselves in the club's first major final of the summer era.
Saints, who finished 16 points clear of second-placed Wigan Warriors on the league table, scored three tries to one, Lachlan Coote converting them all and booting two penalties.
Third-placed Salford had a good spell before the interval, when Jake Bibby scored their lone touchdown and though they offered little with the ball, they produced a stiring defensive effort.
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Saints prop Luke Thompson won the Harry Sunderland award as man of the match.
The final followed the script for the opening 22 minutes with Saints applying constant pressure and scoring two tries, through Morgan Knowles and Zeb Taia.
The first came off a clever pass from Alex Walmsley and James Roby set up the second, following a scrum near Salford's line.
It took Salford 28 minutes to get into Saints' 20 and Tui Lolohea weaved over the line, only for referee Chris Kendall to spot an obstruction by Logan Tomkins on Knowles.
That lifted Reds, though and it was game on 10 minutes before the break when Jackson Hastings played a one-two with Lolohea before firing out a pass to Bibby who got over at the left corner. Krisnan Inu landed an excellent touchline conversion to make it a six-point ball game at half-time.
Salford made a bright start to the second half, but Saints scored on their first attack, 10 minutes in, when Mark Percival grubbered between the posts and ran through to touch down.
Coote's converted and added a penalty on the hour mark after a high tackle by Gil Dudson on Thompson.
That opened a three-score gap and out the game beyond Salford. They struck to their guns, however and the only scores in the final quarter were another Coote penalty - after Salford were offside from Jonny Lomax's kick - and an incredible 40-metre drop goal by Tommy Makinson.