In this paper, we first generalize the LCP of codes over finite fields to the \(\mathbb {F}_2\mathbb {F}_4\) -additive complementary pair ( \(\mathbb {F}_2\mathbb {F}_4\) -ACP) of codes over the mixed alphabet \(\mathbb {F}_2\mathbb {F}_4\) . Then we provide three judging criteria for an \(\mathbb {F}_2\mathbb {F}_4\) -additive code pair (C, D) to be an \(\mathbb {F}_2\mathbb {F}_4\) -ACP of codes. Meanwhile, we also give a sufficient condition for an \(\mathbb {F}_2\mathbb {F}_4\) -additive code pair (C, D) to be an \(\mathbb {F}_2\mathbb {F}_4\) -ACP of codes. In addition, we exhibit properties and characterizations for an \(\mathbb {F}_2\mathbb {F}_4\) -additive code pair (C, D) to be an \(\mathbb {F}_4\) -additive complementary pair ( \(\mathbb {F}_4\) -ACP) of codes. Finally, we provide an interesting application of an \(\mathbb {F}_2\mathbb {F}_4\) -ACP of codes in coding for the two-user binary adder channel.