Ade Akinbiyi has also gone and the board will meet on Monday to discuss a successor, with assistant manager Gus Williams likely to be offered the chance to take over initially.
Sinclair said : “Its a sad day for me because I feel a big part of this club and it has been a pleasure working here, but I just feel I’ve taken the team as far as I possibly can.
“I wanted to build on last season and finish in the top ten this year, but it has been very difficult because of the unexpected departure of so many players in the summer.
“To virtually start again from scratch was a difficult task. I tried to get together a group as good as I could and then look to improve as we went along, but it has come to stage now where I think it is better for the club that they look at somebody else with new ideas and contacts to bring players to the football club.
“The big period for me was November when we didn’t get the points we needed to get us through a tough December and I knew then I was up against it.
“But I feel I can I walk away with my head held high because I’ve given it the best I possibly could.
“I was pitched in as an absolute rookie in management, but it has been a fantastic 23 months for me and I have learnt an awful lot.
“I’ve done some things right and some things wrong, but I am very proud that when I first took the job I was able to turn things round from a desperate situation to win six games in a row at the end of that season and avoid relegation.
“I’ve had some high moments in my career, but that is something I will remember for the rest of my life.”
Frank departs just five games short of celebrating his 100th in charge, and in his time at the club he has made 71 appearances and scored eight goals.
“I would like to thank everybody behind the scenes at the club who have made my stay so enjoyable, from Shelley and her staff who make the food right through to the three chairman I have served under, Bob Paton, Dave Messom and Dave Titchiner, who have been superb,” he added.
“I have only the very best best wishes for the club in the future”.
Frank intends to concentrate on his media work for now, but says: “I’m a football man, its all I know, and hopefully there will be opportunities to perhaps help other clubs in a coaching role at some stage.
“I’ll have a bit of a rest for now, dust myself down and go again.”
Seagulls chairman Dave Titchiner said it has been a very sad day for everyone involved at the club.
“To have someone of Frank’s celebrity status here, along with Ade, has been fantastic for the club and put us on the map in many ways by creating nationwide interest. We thank them for all the work and effort they have put in.”
Sadly it was a disappointing end to Frank’s reign in charge as the depleted side were well beaten in the fog at Boston.
They had to defend for much of the first half and did well to restrict the home side to just one goal, with Rob Hopley clearing off the line, but they conceded two in the space of three minutes early in the second half, and Boston added two further goals in the 70th and 89th minutes.
Paulo Mendes had a goal disallowed for offside for the Seagulls, but they rarely posed a threat in attack.
It was also a sad way for Jonathan Royle to end his time at the club as he now returns to Wrexham, but reserve team player Ashley Seddon came on in the second half for his senior debut and did well.
Colwyn Bay: Sanna, Taylor, Moss, Hopley, Royle, Olapade (Pilliner 57), Haywood, Hodgson, Anoruo, Mendes (Seddon 71), Baynes. Subs not used: Ollerenshaw, Saiel, Akinbiyi.