I was wondering if there is a tool that allows to search for journals where a given researcher is one of the editors.
Try Open Editors, a newly published dataset of ca. 480.000 editorial board positions across 6.000 scholarly journals at 17 academic publishers.
There are no open data* about editorial boards yet.
Thus, research articles that analyze the composition of journals' editorial boards unfortunately collect their data manually. For instance, a paper in 2016 writes at p. 4:
We began with a collection of 605 abbreviated titles of journals [...]. Crowdworkers from Amazon Mechanical Turk then expanded those abbreviations into full journal names and collected the individual editorships associated with each journal.
It has not gotten better since then.
However, there is a reason for optimism as ORCID allows users to list their editorial board positions. Unfortunately, ORCID remains underused; but perhaps in the future its API could offer a greater treasure trove regarding data on journals editors. (The same is true with WikiData).
* Note: By "open data" I mean data that are, inter alia, interoperable, machine-readable, structured - see, for instance, the explanation of "Open Data" at the European Data Portal.