The head of the Maynard retire­ment system pleaded not guilty Tuesday to charges of ­embezzling more than $521,000 from the pension fund, only eight months after pleading guilty to stealing $177,000 from an Asperger’s syndrome charity in a separate case.

Timothy McDaid, 46, of ­Medford, pleaded not guilty in Concord District Court to charges including larceny over $250 and was ordered held on $500,000 cash bail, Middlesex District ­Attorney ­Gerard T. Leone Jr.’s ­office said.

In November, McDaid pleaded guilty to stealing money from the Asperger’s Association of New England and was placed on five years of probation and ordered to pay restitution.

At the November sentencing, McDaid failed to tell prosecutors that he was then also working as the full-time administrator of the Maynard Retirement Board, which handles assets of about $26 million, according to state records. If prosecutors had known of that position, they could have alerted Maynard officials.

McDaid’s attorney in the association’s case, Timothy R. ­Flaherty of Boston, said McDaid is a devoted family man who ­developed an addiction to legally prescribed painkillers that he initially began taking to treat a back injury.

“He struggled with that substance abuse issue, which led to his legal difficulties in the past,” ­Flaherty said.

Jessica Venezia Pastore, a spokeswoman for Leone, said McDaid is accused of stealing the funds from the Maynard system between December 2010 and last month.

She said a system audit conducted several months ago showed financial irregularities, and McDaid allegedly refused to cooperate with auditors.

Prosecutors were told last year that McDaid worked as a contract employee for the Town of ­Maynard, Venezia Pastore said, and authorities should have notified the town of his convictions.

“The connection just was not made in a timely manner,” she said. “We’ve since ­addressed it internally.”

McDaid joined the Maynard board in 2007 and previously worked as the chief auditor of the state Public Employees ­Retirement Administration Commission, which oversees local retirement boards, officials said Tuesday.

Joseph Connarton, the commission’s executive director, said state guidelines bar anyone convicted of a financial crime from acting as a custodian of a public pension.

He said he spoke to ­Maynard pension board members on June 4, soon after the commission learned of ­McDaid’s November convictions. McDaid was then placed on administrative leave from the Maynard board on June 4, Connarton said.

The Maynard Retirement Board said Tuesday in a statement that benefit payments will continue without interruption and that the board, town, and commission “are jointly review­ing all processes and procedures to ensure that the assets” are protected.

McDaid has paid $75,000 of $177,000 in restitution in the Asperger’s syndrome charity case, Venezia Pastore said. He has until November 2014 to pay the remaining balance, accord­ing to court records.