The 38-year-old has played 636 games in his career for various clubs including Sheffield United, where he was a team-mate of Cobblers boss Rob Page.
He was released by Rotherham at the end of last campaign, and replaces previous keeper coach Carl Muggleton, who has left Sixfields Stadium.
Kenny's most recent first-team appearance came for Oldham in 2014.
"Coaching is something I always wanted to do when my playing career ended," Kenny told BBC Radio Northampton.
"I've seen it more over the last few years as I've got older and when I'm training I tend to have more input with the younger lads.
"I'm here as goalkeeper coach. With the new (loan) ruling, I'm registered as a player for the absolute worst case scenario.
"The last couple of years I've not had much game-time and it's gradually brought me down and I've got used to it over the last few years."
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A letter has been signed by more than 150 groups before a European Parliament vote on ways to limit their sale.
Many tech gadgets such as smartphones use conflict materials, so-called because they are mined in warlike conditions and traded by armed groups.
The letter says the law must cover more manufacturers and types of materials, and make firms police supply chains.
European politicians will vote on proposals drawn up by the European Commission on 19 May.
Signed by Amnesty International. Friends of the Earth, Global Witness, Christian Aid, the Ethical Consumer Research Association and many others, the open letter says the trade in conflict minerals fuels wars and human rights abuses around the world.
It says the vote this week is a "landmark opportunity" for Europe to take action to stem the trade and curtail the conflict it engenders.
However, it says the current proposals are "weak" and do not go far enough.
The big problem with the scheme proposed by Brussels is that it applies on a voluntary basis to most companies, the groups argue. As a result, they say, few firms would be motivated to clean up their supply chains and report on their actions.
In addition, the letter says, the scheme only covers four minerals - tin, tantalum, tungsten and gold - in their raw form, and does nothing to address products entering Europe which have been made using conflict minerals. Many smartphones, tablets and other devices made in China are manufactured using these substances.
The letter says the scheme does not cover minerals and materials such as diamonds, jade and chromite, which are also known to be sources of conflict.
It urges MEPs to reject the plans in favour of a stronger scheme involving a mandatory system to ensure firms vet their supply chain.
"Those bearing the cost of our weak efforts to regulate this trade will be some of the poorest and most vulnerable citizens of the world," said the letter. "Now is the time for change."
Romanian MEP Iuliu Winkler - the European Parliament's lead negotiator on the plans - has previously said he "largely supported" the plans, adding that the priority should be ensuring that the right incentives are in place to encourage participation.
However, Green and left-wing MEPs have criticised the opt-in nature of the scheme.

All-rounder Procter, 28, hit 822 runs and two centuries batting at three during 2016 and played in every County Championship game.
Spin bowler Parkinson, 19, made his Red Rose debut in June against Warwickshire where he took a first innings' 5-49.
"I couldn't be happier to sign a new deal. I've really enjoyed my first full season as a professional," he said.
Procter added: "Playing for Lancashire is all I've ever wanted to do, so to sign a new two-year contract is fantastic. I've enjoyed the responsibility of batting at the top of the order this summer."
Veteran goalkeeper Paddy Kenny has joined newly-promoted League One side Northampton Town as player-coach.
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Europe's proposed laws on "conflict minerals" will fail to slow trade in the raw materials, claim rights groups.
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Lancashire pair Luke Procter and Matthew Parkinson have both signed new two-year contracts.