A Worthing win would have all but secured the title, which could then only be lost on goal difference, of which they had a 47 advantage, but the fact that third-placed Brighton 6th beat second-placed Eastbourne 5th 7-2, confirmed Worthing as champions.
Worthing came up against a much stronger Southwick side than they beat 10-0 earlier in the season, as they put together a number of attacks requiring some good Worthing defending.
Dan Lock and Aaron Chapman scored one each in the first half, before a more focused display in the second period.
Chapman bagged another two to make it a hat-trick of hat-tricks so far this season, while Lock managed to net his second.
Worthing captain Dan French, who is now eyeing a 100 per cent record this season, with only four games left, said: "I'm very happy with the team's performance this season with the 100 per cent record so far, league champions and promotion guaranteed.
"What excellent work. But the team will miss loyal centre back Dan Lock as he departs for the Ukraine on Wednesday, after a fitting game to end his season on."
Worthing kept up the pressure at the top of the Kent/Sussex Regional League with a 5-3 home win over BBHC on Saturday.
With table-toppers Lewes and third-placed Holcombe 2nd both winning, Worthing remain two points behind Lewes, having played a game more, and just a point in front of Holcombe, who have played a game fewer.
Rob Lowe and Chris Marr both struck twice for Worthing and debutant Kiwi Tim Smith also netted.
On Sunday, Worthing's mixed side reached the third round of the HA Trophy with a 2-0 home win over South Bucks. Marr and Shelley Formstone netted for Worthing, who will now host Cheltenham in the next round.
Worthing 2nd edged closer to claiming the league title with a 7-1 win over Crowborough in Sussex Area League to open up a six-point lead at the top.
Bryan Lock continued his fine goalscoring form with another hat-trick, and he has now scored in every game he has played this season.
Ellis Woolley, aged 15, made an excellent debut after being called up from the under-16 Tigers side, and netted two goals.
Ahmed Ramiz and stand-in captain John Massie completed the scoring for Worthing.
Local rivals Penguin did Worthing a favour as they held second-placed South Saxons to a 2-2 draw but it was late heart-break for Penguin as Saxons hit an equaliser from a last-minute short corner.
Penguin failed to convert a first-half penalty stroke and Saxons capitalised on missed chances to lead 1-0 at half-time.
Rupert Ireland made up for the early penalty miss by converting a short corner, before Dave Matthews made the most of a defensive lapse to score his 26th goal in 16 games. However, despite all their chances and a strong performance, Penguin had to settle for just a point.
In the same division, Southwick beat Lewes 3rd 3-2, while Worthing 3rd also beat Eastbourne 3rd by the same scoreline.
Colin Warner netted a hat-trick as Littlehampton swept away a determined Worthing 4th side 8-3 to stay five points clear at the top of Division 1 of the Sussex Open League with just four games left to play.
Jon Miles opened the scoring early on with a neat flick from a short corner and Warner soon doubled the lead. Ollie Clarke then hit the third but Worthing got a goal back before half-time.
In the second half, Miles grabbed his second before he crossed for Simon Judd to score.
Steve Jenkins added the sixth before Warner scored from the penalty spot after Sexton was fouled. Warner completed his hat-trick before Worthing got two more goals back.