Newcastle slipped to a timid 2-0 Barclays Premier League defeat with Janmaat sent off before half-time for two bookable offences.
The Dutch defender was booked after twice fouling Jefferson Montero and banished four minutes before the break for pulling back the Ecuadorian winger deep inside the Swansea half.
Newcastle manager Steve McClaren called it a "harsh" decision and felt referee Mike Jones should have shown leniency, but Janmaat was remorseful and apologised to his team-mates and the club's supporters for his red card.
"It was stupid of me and I have to apologise to my team-mates, the supporters and everybody, because trying to win a game with 10 men is very difficult," Janmaat told nufc.co.uk.
SWANSEA CITY 2 NEWCASTLE UTD 0
1. Tim Krul
22. Daryl Janmaat
18. Chancel Mbemba
2. Fabricio Coloccini
19. Massadio Haidara
8. Vurnon Anita
4. Jack Colback
7. Moussa Sissoko 45'
5. Georginio Wijnaldum
14. Gabriel Obertan 82'
9. Papiss Cisse 54'
Subs
6. Mike Williamson
16. Rolando Aarons 54'
17. Ayoze Perez
24. Cheick Tiote
26. Karl Darlow
27. Steven Taylor 45'
45. Aleksandar Mitrovic 82'
Bafetimbi Gomis and Andre Ayew both scored for the second game running as Swansea comfortably saw off 10-man Newcastle 2-0 at the Liberty Stadium.
Gomis opened the scoring in the ninth minute with a composed finish before Ayew, who was also on target in the opening-day 2-2 draw at Chelsea, scored with a powerful second-half header after Newcastle full-back Daryl Janmaat had been sent off for a second bookable offence by referee Mike Jones.
Janmaat was booked after clattering into Jefferson Montero twice and the Dutchman's luck ran out in the 41st-minute when he pulled the Ecuadorian back to stop him launching a Swansea attack from deep.
The defender's departure left Newcastle with a mountain to climb and Steve McClaren's decision to withdraw Moussa Sissoko - the Magpies' best player in the first half - and Papiss Cisse revealed a preference for damage limitation rather than offering any real hope of getting back into this Barclays Premier League contest.
Swansea began with a swagger befitting a side who had held the champions on the opening day of the season, with Jonjo Shelvey and Gylfi Sigurdsson both testing Newcastle goalkeeper Tim Krul from distance.
The breakthrough soon came as Shelvey threaded a delicious ball through for Gomis to race onto.
Newcastle waited in vain for an offside flag as Gomis showed excellent composure to round Krul and score.
Swansea were playing some fluid football but the goal failed to knock Newcastle out of their counter-attacking stride and Lukasz Fabianski had to be alert at his near post to turn Gabriel Obertan's low shot aside.
Sissoko also found Obertan in a threatening position and, although the Frenchman's first touch was heavy, the ball was cut back with only intelligent defending from Federico Fernandez preventing Cisse from equalising.
Newcastle had edged their way back into the contest but things got a whole lot tougher for them when Janmaat saw red for a dozy piece of defending as he pulled back Montero deep inside the Swansea half.
Montero almost immediately capitalised on his extra freedom when he cut inside stand-in right-back Vurnon Anita and Sissoko to thud the Newcastle post from 20 yards.
Swansea struck the woodwork again within three minutes of the re-start when Sigurdsson hit the underside of the bar from 20 yards but the second goal soon came.
This time Montero raced beyond Chancel Mbema and his cross was met by the powerful head of Ayew, who gave Krul no chance to register his second goal in English football after his debut strike at Chelsea.
Ayew almost added another but Swansea were in cruise control as they made it six wins out of nine against Newcastle in the Premier League.
THE TOON'S GREATEST 100 PLAYERS ... EVER!
AMAZON: http://www.amazon.co.uk/The-Toons-Greatest-Players-EVER/dp/132629685X
This book is a collection of the 100 Greatest Newcastle United players of all time, as voted for by NewcastleUtd-Mad.co.uk readers and a panel of journalists who contribute to the website. The players are in an order of five-per-chapter, counting down from 100 to Newcastle United's greatest ever player (as viewed by the author), but the whole point of the book is for YOU (the reader) to decide who goes where. The list is not, by any means, definitive. As with all books of this nature, whether it be "Greatest Albums"; "Best Movies Of All Time"; or "Top 100 Pin-Ups Of The Year" ... no two people have exactly the same opinion. There will be controversy over who is on the list, and chances are, more controversy over who is not. Here are the best of the best. I have painted the picture with each player's profile. Where they end up in the league of NUFC's Greatest is up to you. Because YOU have the final word.
Otherwise ... if you want a signed copy (for a birthday, etc) for £9.95 and no postage ... you can meet me before a match and I will bring a copy along.
Get in touch: Kevnumad@fsmail.net
NEWCASTLE UNITED
Fri 10th July (8pm BST)
Gateshead FC 0 Newcastle Utd 1 (Cisse pen)
(First half): Darlow, Janmaat, Haidara, Lascelles, Williamson, Anita, Colback, Gouffran, Cisse, Sissoko, De Jong. (Second half): Woodman, Bigirimana, Satka, Good, Ferguson, Tiote, Obertan, Aarons, Perez, Armstrong, Riviere.
Tue 14th July (8pm CT)
Club Atlas (MEX) 2 Newcastle Utd 1 (Sissoko)
Krul (Darlow 46); Janmaat (Bigirimana 72), Williamson, Lascelles, Haidara (Ferguson 72); Anita, Colback; Sissoko (Obertan 72), de Jong (Perez 72), Gouffran (Aarons 72); Cisse (Armstrong 72).
Sat 18th July (8pm UTC)
Sacramento Republic FC (USA) 0 Newcastle Utd 1 (og)
Krul (Woodman 46), Janmaat, Williamson, Lascelles (Taylor 46), Haïdara, Anita, Colback, Sissoko, Obertan (Aarons 69), de Jong (Perez 69), Cissé (Wijnaldum 69).
Tue 21st July (7.30pm UTC)
Portland Timbers (USA) 4 Newcastle Utd 3 (Wijnaldum, Aarons 2)
Darlow (Woodman 46), Bigirimana, Satka, Ferguson, Aarons; Abeid (Anita 65), Wijnaldum (Janmaat 46), Gouffran (De Jong 76), Sissoko (Obertan 46), Ayoze, Cisse (Armstrong 46).
Sun 26th July (1pm BST)
Sheffield United 2 Newcastle Utd 2 (Wijnaldum, de Jong)
Krul, Janmaat, Williamson, Taylor (Lascelles 69), Haïdara, Anita (Obertan 59), Colback, Sissoko (Abeid 80), Wijnaldum (De Jong 73), Perez (Aarons 45), Cissé (Armstrong 80)
Wed 29th July (7.45pm BST)
York City 2 Newcastle Utd 1 (Perez)
Darlow (Elliot HT); Bigirimana, Lascelles, Satka, Ferguson; Abeid, Gouffran; Aarons (Kemen 59), De Jong, Perez; Mitrovic (Suddick 70)
Sat 1st August (3pm BST)
Newcastle Utd 0 Borussia Mönchengladbach 1
Krul, Janmaat (Mbemba 62), Williamson, Taylor, Haïdara, Anita (De Jong 75), Colback, Sissoko (Perez 81), Obertan (Aarons 62), Wijnaldum, Cissé (Mitrovic 62).
PREMIER LEAGUE
Sunday August 9
NEWCASTLE UTD 2 SOUTHAMPTON 2 (Cisse, Wijnaldum)
Krul, Janmaat, Mbemba, Coloccini, Haidara; Colback, Anita (Tiote, 67); Sissoko, Wijnaldum (De Jong, 81), Obertan; Cisse (Mitrovic, 75). Subs not used: Darlow, Taylor, Aarons, Perez.
Saturday August 15
SWANSEA CITY 2 NEWCASTLE UTD 0
Krul, Janmaat, Mbemba, Coloccini, Haidara, Anita, Colback, Sissoko (45' Taylor), Wijnaldum, Obertan (82' Mitrovic), Cisse (54' Aarons). Subs: Williamson, Perez, Tiote, Darlow
Saturday August 22
12:45 Man Utd v Newcastle United
Saturday August 29
12:45 Newcastle United v Arsenal
Monday September 14
20:00 West Ham United v Newcastle United
Saturday September 19
15:00 Newcastle United v Watford
Saturday September 26
17:30 Newcastle United v Chelsea