We construct a class of $\mathbb{Z}_2\mathbb{Z}_4$-additive cyclic codes generated by pairs of polynomials, study their algebraic structures, and obtain the generator matrix of any code in the class. Using a probabilistic method, we prove that, for any positive real number $δ<1/3$ such that the entropy at $3δ/2$ is less than $1/2$, the probability that the relative minimal distance of a random code in the class is greater than $δ$ is almost $1$; and the probability that the rate of the random code equals to $1/3$ is also almost $1$. As an obvious consequence, the $\mathbb{Z}_2\mathbb{Z}_4$-additive cyclic codes are asymptotically good.