Merrick, who is in his second season at the club, said he had thoroughly enjoyed the last 20 months with Wellington and it was an easy decision to commit himself for the long term.
"I wasn't going to speak to any club without talking with (Phoenix chairman) Rob Morrison first of all, and the Phoenix," he said.
"It was a two-week process, two discussions and a phone call - it was very straightforward.
“The club is stable, financially sound and ambitious,.
“The energetic staff are great to work with and we have a great squad of players.
“The brand of football we are playing is a credit to the players, and the young guns coming through our reserve team and Academy will add to the attacking flair.
“We have something special here in Wellington and I want to continue to be a part of it.
“I appreciate the opportunity I have been given by the board.”
Last in 2013 and ninth last year, Phoenix is fourth on the A-League ladder, just three points off leader Perth Glory with eight games remaining.
Greenacre, a popular player with Phoenix fans before turning his hand to coaching, said he was absolutely delighted to be given a long-term deal.
“I am also so pleased that Ernie is staying for that time because it has been great working with him,” Greenacre said.
“Everyone on the staff has learnt from him and hopefully I’ve proved my worth as a coach.”
Greenacre admitted that it was hard to hang up his boots but has never regretted it.
“All players wish they could play on forever but the reality is that you have to stop some time,” he said.
“It was a career choice and hopefully I am on a long road in coaching.”
Phoenix chairman Rob Morrison said the two signings were significant for the club.
“One of the key objectives for the board and the ownership group was to create a stable operating platform for the club,” Morrison said.
“Without stability off the field you cannot hope to be successful on the field, so the board is delighted that Ernie and Chris have bought into the club’s vision and signed long-term contracts.
“Ernie has been instrumental in establishing the attractive, possession-based style of attacking football that we wanted when we first signed him.
“His influence throughout the club has been immense, so keeping Ernie at the club was a priority for the board and having done so we all look forward to exciting times ahead, not least of which is the run into the finals this season.”
