The former Manchester United manager, who is due to take up a teaching position at Harvard Business School this month, is among several investors who have pumped a total of £1 million into the firm ahead of a consumer launch.
Mark Newton-Jones, interim chief executive of Mothercare and formerly of Shops Direct Group, has joined Sir Alex in taking a "minority but meaningful" stake.
The move by Sir Alex, who retired from full-time management last May, comes as he prepares to auction 5000 bottles of wine in Hong Kong and London.
Denis Shafranik, Simon Barklem and Ian Langley are among other external investors who have taken part in the funding round, which follows the investment of an initial £1 million in seed funding from Danny Jatania, co-founder of the Lornamead Group.
Prior to the latest investment Pockit strengthened its management team and invested in a new website in readiness for a major marketing drive.
Sir Alex said: "I am excited by the plans for Pockit, their management team is superbly placed as the Jatania family has an outstanding track record of building business from the ground up and growing brands. I am looking forward to watching the business develop."
Pockit, whose card is issued under the Mastercard brand, allows consumers with no bank account or poor credit ratings to enjoy the flexibility of paying by card, including online shopping.
Although holders are charged 45p per transaction, people can spend only the money they load on to it, and are not allowed to become overdrawn.