
die Mehrarbeit
overtime
Work performed beyond the contractually agreed or usual working hours; overtime.
Wegen des dringenden Projekts mussten alle Mitarbeiter Mehrarbeit leisten.
Because of the urgent project, all employees had to work overtime.
overtime / extra work
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overtime
Work performed beyond the contractually agreed or usual working hours; overtime.
Wegen des dringenden Projekts mussten alle Mitarbeiter Mehrarbeit leisten.
Because of the urgent project, all employees had to work overtime.

extra work
The additional effort or extra work required to complete a task compared with the original plan.
Die nachträglichen Änderungswünsche des Kunden verursachten erhebliche Mehrarbeit.
The customer's later requests for changes caused considerable extra work.
The word is simple in structure and easy to understand directly: it is made up of 'mehr' (more) and 'Arbeit' (work). So it literally means 'additional work'.
Jede Änderung am Plan bedeutet Mehrarbeit für das gesamte Team.
Every change to the plan means extra work for the entire team.
'Mehrarbeit' (overtime / extra work) is almost always used as an uncountable noun in the singular. You say 'viel Mehrarbeit' (a lot of extra work), but normally not 'viele Mehrarbeiten'.
Durch die Krankheit meiner Kollegin habe ich gerade sehr viel Mehrarbeit.
Because my colleague is sick, I currently have a lot of extra work.
The exact meaning depends on the context. If it is about paid working time and overtime hours, the first meaning is intended. If it is generally about additional effort for a task, the second meaning is intended.
Die Firma muss die geleistete Mehrarbeit extra bezahlen.
The company has to pay extra for the overtime worked.
In a professional context, 'Mehrarbeit' (overtime) is often used with the verb 'leisten' (to perform/do). A supervisor can also 'anordnen' (order) or 'anweisen' (instruct) overtime.
Die Mitarbeiter waren bereit, für das Projekt unbezahlte Mehrarbeit zu leisten.
The employees were willing to work unpaid overtime for the project.
When talking about additional effort, people often say that something 'Mehrarbeit verursacht' (causes extra work) or 'zu Mehrarbeit führt' (leads to extra work). It is often combined with adjectives like 'erheblich' (considerable) or 'beträchtlich' (substantial).
Die unklaren Anweisungen haben zu erheblicher Mehrarbeit geführt.
The unclear instructions led to considerable extra work.
overtime
Both words refer to work beyond the agreed or usual working hours. An „Überstunde“ is one additional working hour and can be counted. „Mehrarbeit“ refers to additional work or working time overall, without presenting it as individual hours.

Wegen des dringenden Auftrags fällt für Jana diese Woche Mehrarbeit an.
Because of the urgent order, Jana has extra work this week.
Wegen des dringenden Auftrags macht Jana heute eine Überstunde.
Because of the urgent order, Jana is working one hour of overtime today.