Mansor, 68, has been charged with accepting three bribes, worth a total of 194 million ringgit ($47 million) in 2016 in exchange for helping a private company secure the contract of a solar energy project for 369 rural schools in the state of Sarawak, situated in the Borneo province. If found guilty, she will be jailed for 20 years.
The company, Jepak Holdings, had secured the "Project Bersepadu System Solar Photovoltaic (PV) Hybrid" - valued at 1.25 billion ringgit - without an open tender.
In the first hearing of the trial in the Kuala Lumpur High Court - postponed on Monday after Mansor skipped the court appearance citing medical problems - two of the prosecution witnesses began testifying, with prosecutors aiming to present direct evidence of the alleged crimes committed by the former first lady.
"The accused is the wife of the former Prime Minister. By herself, she occupied no official position. However, she wielded considerable influence by reason of her overbearing nature," prosecutor Gopal Sri Ram said in his opening statement.
"She placed herself in a position where she was able to influence decisions in the public sector," he added.
Mansor, who also faces 17 other allegations of corruption, has pleaded not guilty to all charges and is out on bail.
Razak, who faces 42 corruption charges, and his wife are accused of siphoning millions of dollars to their private accounts from the state fund 1Malaysia Development Berhad (1MDB).
The 66-year-old leader - who was the prime minister between 2009 and 2018 - allegedly appropriated about 2.6 billion ringgit, according to a media investigation carried out in 2015.
Three of the five proposed trials against Razak are already underway over charges of corruption, abuse of power and money laundering.
When the 1MDB scandal first grabbed headlines during his tenure, Razak had claimed that the money was a donation from a Saudi prince, an explanation that was initially accepted by the 1MDB and also by the Anti-Corruption Commission and the Attorney General, who had exonerated him from the charges.
However, current Prime Minister Mahathir Mohamad - a former mentor of Razak - reopened the investigation and pressed charges against his erstwhile protege after a surprise win in May 2018 elections.
In addition to Malaysia, half a dozen countries, including the United States, Switzerland and Singapore, continue to investigate the embezzlement of the Malaysian state fund.
The US Department of Justice estimated $4.5 billion had been diverted from 1MDB of which about $1 billion may have been laundered in the US through the purchase of real estate, yachts, jewelry and works of art, among other goods.
( With inputs from IANS )