The tech firm revealed the change in a blog at the end of last week.
Google promised to make the move before the year's end to bring the consumer version of Gmail in line with its business edition.
The firm had faced much criticism over the years for the scans.
The measure helped justify the cost of offering the public one gigabyte of "free" webmail storage in 2004 - an offer that was so much greater than the competition at the time that many originally believed it to be a joke.
However, UK-based campaign group Privacy International tried to block the scans once it became apparent they were the cost of signing up to the service. The organization tried and failed to get the country's data privacy regulator to intervene.
Then, a decade later, Microsoft ran a series of adverts, in which it first depicted a "Gmail man" searching through people's messages, and then went on to accuse the search giant of "crossing the line" causing its customers to be "Scroogled".
"When they first came up with the dangerous idea of monetising the content of our communications, Privacy International warned Google against setting the precedent of breaking the confidentiality of messages for the sake of additional income," the charity's executive director Dr Gus Hosein told the BBC.
"Of course they can now take this decision after they have consolidated their position in the marketplace as the aggregator of nearly all the data on internet usage, aside from the other giant, Facebook.
"The reality is that what you choose to say over email to another human being isn't as interesting for exploitation as the data you have no control over - Google would rather exploit your data by tracking you across the internet, across their mobile operating system, their search engine, their apps, their smart devices, and likely someday soon, their car, amongst a myriad of other services that they dominate through the exploitation of our data."
Google's blog notes that users can opt out of seeing personalised ads on any of its services by changing their account settings.
Another digital rights body, Big Brother Watch, was a little more positive.
"Whilst it could be seen as closing the stable door once the horse has bolted, there is no doubt that the end to the intrusive and frankly creepy process will be appreciated by a great many Gmail users," commented its chief executive Renate Samson.
"However, none of us must rest on our laurels.
"Whilst tech companies should see this as an opportunity to halt other intrusive snooping for advertising purposes, citizens equally should take greater care not to sign up to services which routinely share your personal information with third parties for the purposes of advertising or marketing.
"Google's move is absolutely a step in the right direction, let's hope it encourages others to follow suit."
While the ad-driven scans should soon stop, the news site Ars Technica has highlighted that Gmail messages will still be scanned by Google to provide artificial intelligence-powered "smart replies", malware-protection and sorting for search queries.
Photo: Microsoft image (Microsoft had attacked Google's email scans claiming they were an invasion of privacy)