Nagai played GeGeGe no Kitarō's Konaki Jijii, a comic, absent-minded old yōkai man who attacks enemies by clinging to them and turning himself to stone, increasing his weight and mass immensely and pinning them down between the 60s and 80s alongside Masako Nozawa and Keiko Toda (Kitaro), Isamu Tanonaka (Medama oyaji), Chikao Ohtsuka and Kei Tomiyama (Nezumi-Otoko), Nana Yamaguchi, Yūko Mita (Neko Musume), Yoko Ogushi, Keiko Yamamoto (Sunakake Baba), Yonehiko Kitagawa, Kenji Utsumi and Yusaku Yara (Nurikabe) and Jōji Yanami, Keaton Yamada and Kōsei Tomita (Ittan Momen).

Nagai was cast as Sazae-san's character, Namihei Isono, Sazae's father and patriarch of the family in the longest running Anime in 1969 alongside co-stars Midori Katō (Sazae) and Miyoko Asō (Fune) until Chafurin filled in for him after his death.

He was cast in Urusei Yatsura as the wandering monk Cherry in 1983 alongside co-stars Toshio Furukawa (Ataru Moroboshi), Saeko Shimazu (Shinobu Miyake), Fumi Hirano (Lum) and Akira Kamiya (Shutaro Mendo).

He often alternates between his role as a villainous grandmaster to one as a lighthearted pervert, alongside co-stars Kappei Yamaguchi and Megumi Hayashibara (Ranma Saotome), Kenichi Ogata (Genma), Masako Ikeda (Nodoka), Noriko Hidaka (Akane Tendo), Aso (Cologne), Hirotaka Suzuoki and Kōji Tsujitani (Tatewaki Kuno), Koichi Yamadera (Ryoga Hibiki) and Hiromi Tsuru (Ukyo Kuonji).

On January 27, 2014, while recording narration for a program in Hiroshima, Nagai suffered a bout of myocardial infarction and was found by a hotel employee.