The life member of Bondi Surf Club set off from England early Wednesday morning local time and arrived at Cap Gris Nez in France some 12 hours and 45 minutes later.
"I made it!" Baldock tweeted immediately after completing the 34km marathon.
"I'm officially the oldest person to swim the English Channel."
Baldock, aged 70 years and nine months, has replaced Englishman Roger Allsopp in the record books.
Allsopp was 70 years and four months when he took almost 18 hours to cross the Channel in 2011.
"I haven't had that much fun in years," Baldock said at Cap Gris Nez.
"At the 10-hour mark I felt like I was going nowhere but my support crew were fantastic in their encouragement and inspired me to make it to the finish."
Baldock knew how tough the long-distance swim would be having previously crossed the Channel in 1985 as a 41-year-old.
Back then the feat took him 10 hours and 44 minutes.
Halfway through Wednesday's swim the Bondi local's support crew tweeted that he was taking on sports drinks and an anti-inflammatory drug.
With the French coastline in sight the team revealed the 70-year-old was "starting to hurt but still enjoying it".
Baldock was fulfilling a decades-old promise to his former coach Des Renford by completing the crossing. He was also raising money for cancer research.
Australian Trent Grimsey holds the record for an unassisted solo crossing of the English Channel having completed the marathon in six hours 55 minutes flat in 2012.
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