The two players will take up their positions on the PCA board in support of the chairman Daryl Mitchell, with a remit to champion the ongoing interests of professional cricketers in England and Wales.
"I am delighted to welcome both Heather and James onto the PCA Board," said Mitchell. "Both bring vast amounts of experience and knowledge across the professional game and I have no doubt they will make the PCA even stronger."
Knight, 29, has been England women's captain since 2016, including the World Cup final victory over India at Lord's in 2017. She has also played a key part in establishing the England Women's Player Partnership and remains on the EWPP Committee to ensure contractual and commercial rights of players are protected.
"The women's game is growing so quickly," said Knight. "I think it is really important to have a voice on there for the women's game, not just for the international players, but for the domestic players that are joining the professional game in the next few months.
"I have always been interested in how the game works and what the right decisions are for the wider game so I'm very excited by the opportunity to work closely with the PCA to continue the momentum we've seen in women's cricket over the last few years.
Harris, meanwhile, has been a professional cricketer since breaking into the Glamorgan team at the age of 16 in 2007, and joined the Players' Committee in 2017. He has since moved on to play for Middlesex, and has taken 620 wickets in 261 matches.
"I am honoured to be given this opportunity," said Harris. "I've been doing more and more work with the PCA over the last few years and helping Daryl out with any extra support he has needed as part of my role on the Players' Committee.
"Daryl has had an incredible amount on his plate with the current pandemic and everything that has preceded this during his time as chairman, including the new standard playing contract which he has played such a huge part in over the last three years. With this role becoming official I will hopefully be able to help him out even more."
Both players will join the board with immediate effect, and attend their first board meeting in mid-July.
"The role of vice-chairs is an important one to add further structure to the PCA Players' Committee," said Mitchell. "My first three years as Chairman has coincided with enormous change within the game due to the new broadcast deal and the subsequent new white ball tournament and County Partnership Agreement.
"While I have thoroughly enjoyed the responsibility and the challenge, it has become apparent two vice-chairs sitting above the Committee can really help fulfil roles to support our association.
"More current player representation on the PCA Board is also a progressive move to ensure players views are at the forefront of shaping the future of professional cricket careers and the views of the PCA."