Brazilian striker Leo Ceara scored in the 20th and 57th minutes to allow Kashima to claim the title by a point from their nearest rivals and earn the club a record-extending ninth J-League crown.
The title is the fifth for manager Toru Oniki, who joined Kashima at the start of the season, having previously won the league on four occasions with Kawasaki Frontale.
Kashima, who led Kashiwa by a point going into the 38th and final round of fixtures, went into the break ahead when Ceara, the season's leading scorer, volleyed in from seven yards out.
The 30-year-old netted his 21st goal of the campaign to double Kashima's lead 12 minutes into the second half, heading in Yuta Matsumura's clipped cross from the right.
That goal was to prove crucial as Marinos' Jun Amano chipped the ball over goalkeeper Tomoki Hayakawa a minute into added time to heighten the tension late in the game, but Kashima held on to seal their title win.
Kashiwa confirmed their second-place finish with a 1-0 win over Machida Zelvia through an own goal by Daihachi Okamura, with Kyoto Sanga beating deposed champions Vissel Kobe 2-0 to take third.