徳島県徳島市出身。1970年にグラフィックデザイナーとして活動を開始。1973年の夏にアメリカを40日間旅し、翌1974年にはグアム島を訪れて風景に深い感銘を受け、後の作風の原点となる。1975年からイラストレーター湯村輝彦に師事し、1978年に独立。1980年代以降は、大瀧詠一の『A LONG VACATION』『NIAGARA SONG BOOK』をはじめとする、透明感あふれるトロピカルな風景イラストで知られるようになる。『A LONG VACATION』のジャケットはCBSソニーから「アルバムジャケット特別賞」としてゴールドディスクを受賞した。

大瀧との仕事をきっかけに独自の地位を確立し、その後もサザンオールスターズ、松岡直也、石黒ケイ、藤原ヒロシ&川辺ヒロシ、憂歌団、杉山清貴、西城秀樹など、数多くのアーティストのレコードやCDジャケットを手がける。近年では、AORコンピレーション『breeze』シリーズ、ikkubaru『Amusement Park』『brighter』、TEEN RUNNINGS『NOW』、サニーデイ・サービス『DANCE TO YOU』のアートワークを担当。さらにZoffの2016年サングラスキャンペーンやSUBARU WRX S4のテレビCMにもイラストが起用された。

展覧会活動も積極的で、2014年には「HIROSHI NAGAI EXHIBITION 永井博アート展」(東急百貨店本店)、2016年には東京「CAY」と大阪「digmeout ART & DINER」で個展「Penguin's Vacation Restaurant」を開催している。

Born in Tokushima City, Tokushima Prefecture. He began working as a graphic designer in 1970. In the summer of 1973, he traveled to the United States for 40 days, and the following year, in 1974, he visited Guam, where he was deeply impressed by the landscape, which became the starting point for his later style. He began studying under illustrator Teruhiko Yumura in 1975 and went independent in 1978. Since the 1980s, he has become known for his crystalline tropical landscape illustrations, including for Eiichi Ohtaki's "A LONG VACATION" and "NIAGARA SONG BOOK." The cover of "A LONG VACATION" won a gold disc from CBS Sony as a "Special Album Cover Award."

His work with Ohtaki established him as a unique artist, and he has since created record and CD covers for numerous artists, including Southern All Stars, Naoya Matsuoka, Kei Ishiguro, Hiroshi Fujiwara & Hiroshi Kawabe, Yuugadan, Kiyotaka Sugiyama, and Hideki Saijo. In recent years, he has been responsible for the artwork for the AOR compilation series "breeze," ikkubaru's "Amusement Park" and "brighter," TEEN RUNNINGS' "NOW," and Sunny Day Service's "DANCE TO YOU." His illustrations have also been used in Zoff's 2016 sunglasses campaign and a television commercial for the SUBARU WRX S4.

He is also an active exhibition organizer, having held the "HIROSHI NAGAI EXHIBITION: Nagai Hiroshi Art Exhibition" (Tokyu Department Store Main Branch) in 2014 and the solo exhibition "Penguin's Vacation Restaurant" at "CAY" in Tokyo and "digmeout ART & DINER" in Osaka in 2016.