Nicola Richards, who was a counting agent at the elections for the region's mayor and police and crime commissioner, said she was "frankly appalled" at how biased presiding officers were at the count.
She said votes which should have been thrown out were counted for Labour, including one paper where the only mark was the party emblem "scribbled out".
Both elections went to a second round of voting. Conservative Andy Street won the mayoral election despite losing in Sandwell by more than 6,000 votes, while Labour's Simon Foster won the PCC election, finishing around 13,500 votes ahead in the borough.
Ms Richards, the MP for West Bromwich East, detailed her complaints in a letter to the Electoral Commission.
She wrote: "I was quite frankly appalled at how biased the presiding officers were when conducting themselves at the count.
"Firstly, the deputy returning officer discounted a handful of Conservative votes on the basis that there was a scribble in the box as opposed to a cross.
"I did not disagree with this, but I was very shocked to see that a vote was counted for Labour where the emblem was scribbled out with no other discernible marks on the ballot paper.
"Other agents and I asked to re-examine every vote that had previously been rejected for 'scribbles' but the presiding officer refused.
"All that I asked for was consistency in the process."
She adds that when the doubtful and spoilt ballots were checked for the second round, it became clear that "tens of votes" had gone through which should have been rejected in the first round.
"Upon being challenged by myself and others, the returning officers said they were 'sorry' that this had happened, but there was nothing that they could do.
"I don't think it's acceptable that clearly spoilt ballots were passed through to the second round."
Ms Richards also claims election agents at the count were "learning on the job" and had not been properly trained.
"I must ask in the strongest possible terms that the Electoral Commission dispatch independent inspectors to future counts at Sandwell MBC," she wrote.
A Sandwell Council spokesperson said: “The returning officer and their deputies follow the guidance provided by the Electoral Commission when adjudicating doubtful ballot papers, and all adjudication is carried out in front of observers, including candidates, election agents and count agents.
"If an observer disagrees with a decision by the returning officer or their deputies to reject a ballot paper, then they can ask the returning officer to mark on the ballot paper 'rejection objected to'.
“Each returning officer (or authorised deputy returning officer) is ultimately responsible for making a decision on individual ballot papers.
"Their decision to reject a particular ballot paper during the count is final and can be reviewed only following an election petition after the declaration of the result.
"An election petition must be presented to the court within 21 calendar days after the date of the election. No such petition has been presented following any of the polls held in Sandwell on May 6, 2021.”