The recent works \cite{EFMV2024} and \cite{JOP2023} have studied the spectral properties of the Dyson model in the absence of an external field. This paper is a continuation of \cite{EFMV2024} and aims to bridge the gap in the literature by investigating the Dyson model in a field.\\ In this paper, we prove that, for high temperatures or strong magnetic fields, there exists a non-negative, integrable (with respect to the unique half-line Gibbs measure) eigenfunction of the transfer operator for the Dyson model if $α\in(\frac 3 2,2]$. However, unlike in the zero-magnetic-field case, this eigenfunction is not continuous.