The show debuted last Friday at 9pm on Five without sponsorship, opening the 23-part second series with 2.4 million viewers.
The deal was agreed with too tight a deadline to get the creative ready for the first show.
Lucozade brand manager Alex Saunders said: "It was agreed that we would leave the idents off the first epiode as we wouldn't be able to get everything ready in time."
It is the first TV sponsorship for Lucozade Alert Plus, an energy shot launched in September by GlaxoSmithKline.
Each sponsorship spot features a bottle of Lucozade Alert Plus fizzing with electricity. The electric current transforms into a word that completes a number of sentences, such as 'The Mentalist has clarity', 'The Mentalist has insight' and 'The Mentalist is sharp'.
The first series of 'The Mentalist' proved a hit for Five last year, bringing in an average weekly audience of 3.5 million viewers.
The sponsorship deal was brokered by Five's sponsorship controller Rachael Wheeler. M&C Saatchi created the sponsorship assets including a 15-second opening credit, six five-second break bumpers and a 10-second closing credit.
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