The MP for Peterborough, who was expelled by the Labour Party, was driven out of HMP Bronzefield in Ashford in Surrey on Tuesday morning.
She was jailed for three months at the Old Bailey on January 29 after being convicted of perverting the course of justice by lying to avoid a speeding charge.
Onasanya, 35, sat in the back of a white Sodexo-branded Vauxhall with the closest window to her covered by a hi-vis jacket as it drove past photographers at speed.
The MP with a once-promising career is continuing in her role, sitting as an Independent politician despite calls for her resignation.
She is also appealing against her conviction.
A jail sentence of at least twelve months is required for the automatic removal of an MP under parliamentary rules.
But a lesser sentence can result in a recall petition, which would result in a by-election if it is signed by more than ten per cent of the electorate in the constituency.
Onasanya, a solicitor by profession, is expected to have to obey strict conditions as part of her early release.
It came after the Attorney General's Office ruled that her sentence was not unduly lenient following a complaint being lodged.
Jurors at the Old Bailey were told that Onasanya colluded with her brother Festus after her car was clocked speeding at 41mph in a 30mph zone in the village of Thorney, near Peterborough, in July 2017.
She was sent a Notice of Intended Prosecution to fill out, but it was sent back naming the guilty driver as Aleks Antipow, an acquaintance of her brother, who was away visiting his parents in Russia.
Festus Onasanya was sentenced to ten months in prison after he pleaded guilty to three counts of perverting the course of justice over speeding, including over the 24th July incident.
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