In a new video statement, Baroness Hallet said from August to December 2020, Wales had the "highest age standardised mortality rate of the four nations".
"It's likely that this was a result of a combination of failed local restrictions, a firebreak lockdown that was imposed too late despite exponential growth of the virus, and the decision to relax measures more quickly than scientists advised."
She said that by late 2020 Wales, as well as the rest of the UK, "failed to take decisive action in response to the entirely foreseeable Alpha variant".
"Instead, it pressed on with plans for relaxing measures over Christmas, while cases grew rapidly, only to change course on the 19 December, when levels of infection became critical," she said.
Baroness Hallett added that the "mistakes" of February and March 2020 were repeated and "a return to lockdown had once again become unavoidable".
"That was inexcusable."
Originally the Welsh government had planned to relax rules from 23 to 27 December 2020 to allow people to celebrate with loved-ones but these were scrapped on 19 December 2020 with meeting up limited to Christmas Day.
In announcing the measures, which meant people had to stay at home and only go out for essential reasons, Drakeford said hundreds of people had contracted a new, "more aggressive" variant of the virus in Wales.
Similar measures had also been introduced by the then Prime Minister Boris Johnson in parts of England.